What you need is chloramphenicol eye drops or gel to prevent infection. There is nothing wrong with putting a COOL compress on your guinea pig to reduce swelling. You can use a soft clean towel, sterile guaze is not necessary. Go to the pharmacy and get artificial tears, which is made for people. Twice a day, rinse her eye with this. Simply hold her in your lap, tilt her head so you can see her eye, and gently squirt it with the artificial tears. Now, a CHEAPER way to do the same thing is get the wetting solution for soft contact lens wearers. You want the saline solution, not the cleaning solution. This is what I use for my guinea pigs because I tend to squirt in quite a bit and you get more for your money than a small bottle of artificial tears. Also, contact lens solution has to meet certain standards and artificial tears are unregulated. But if you want artificial tears, get a brand made in the UK, EU, or USA from a reputable pharmaceutical company. There was a serious problem in the USA with a brand imported from Asia and sold under the name of a local pharmacy.
Now, the eye has a wonderful way of healing itself in many cases. Our eyes encounter a lot of dirt in everyday life. So, if your guinea pig has clear eyes (the white part is white (not red) and the middle of the eye is the normal color (not cloudy), then you simply need to make sure the swollen eyelid gets better in a couple days. If not, then you need to take advantage of the local veterinarian who has emergency hours, if you can't get an appointment with a vet during the day.