Thank you for the welcome! She is an only guinea pig, yes...(The store that I bought her from actually told me not to get another. She was in the adoption centre as she had been treated for ringworm and when they tried to integrate her back, it wasn't successful. Having read online/ research, I am slightly worried as it does say everywhere to keep them in pairs but the store have still said not to...A little bit stuck, to be honest!). She was originally sold as a boy (her name is Forrest Gump haha) but when the store member was demonstrating a health check, I pointed out the 'nipples' and they then said it was a little girl, I have just found out on this forum that both boys and girls have nipples so I am going to check her downstairs asap!
Please do! Rumblestrutting is very much typical boar behaviour. Sows generally do it only when they come into season.
Please feel for the presence/absence of a penis ridge under skin and the presence/absence of a fleshy seal inside the slit (the photos in the guide tell you exactly where to look and how the slit insides should look). If the answer is coming back the same from each area, you know that you have the correct gender. These areas are much clearer than just looking at the arrangement of the genitalia, which is why mis-sexing is so common. If necessary, ask somebody else to hold your piggy for you.
I guess that also should answer your question as to how much the pet shop staff REALLY know about the pets they are selling, shouldn't it?
However, please don't run out and get another one from the same place straight away!
Firstly you need make sure that any ringworm spores still potentially sitting in the coat have gone (I had ringworm carried in twice that way) by giving your piggy a fungal bath. We normally don't recommend baths, but in this case it is very important and can save you some really messes down the line! If you have an ex-ringworm piggy, getting hold of some good vet grade disinfectant (brand names in the guide) is one of the best investments you can ever make. You can use the disinfectant for a deep clean every 3 or 6 months anyway.
When you bathe, please do so in a bowl sitting on a towel on the bottom of a shower or bathtub so your piggy cannot injure itself if it blind jumps, and can also not escape. have another dry towel waiting. any water should not be higher than 1 inch and should be baby warm. For a one-off bath, nizoral or any dandruff shampoo from a pharmacy will do the trick.
What to check and look out for in new guinea pigs (vet checks, sexing, parasites&illness)
Ringworm: Hygiene And Pictures
Before you get another one please do your research re. cage sizes. Pet shops will sell anything to get repeat customers for food and bedding. Most cages are too small (but generally overpriced). Buying a small pet is by far the cheapest bit. The most expensive is vet care.
If you have a good standard safe rescue you can get to, please contact them for a dating slot on their adoption days. At least you can be sure that way that you will adopt only a healthy/quarantine and vet treated, properly sexed piggy and that you come home with a new same sex mate only if acceptance has happened, which a goes a long way towards a stable bond. A few US rescues even have spayed sows.
UK:
Recommended Guinea Pig Rescues
US and some other countries:
Guinea Lynx :: Rescue Organizations
Please take the time to do your research first before you get a second guinea pig, but also be aware that guinea pigs are social animals that live in groups and that are not really wired to be all on their own. But they are also great individual personalities, so it is not a matter of simply sticking another piggy into the cage.
You can find lots of good and helpful information via this link here, but are welcome to ask any further questions you have in our Care sections:
Getting Started - New Owners' Most Helpful Guides
to the forum! I hope you are planning on getting another pig of the same sex soon (or a neutered boar), as guinea pigs need to live in pairs in order to be happy/healthy! 
