Sometimes a lump on the nipple isn't an infection but if he had antibiotics most of them taste bitter and he won't like them. If it's not an infection it won't matter anyway. If he is taking a painkiller called metacam or loxicom (with active ingredient meloxicam) this tastes sweet and they usually love it and will try and grab the syringe! Sometimes pigs can develop a mammary tumour - but this isn't always cancer, it can be benign - but tumours can all grow and get very big. Sometimes it isn't a tumour but just a fatty lump which is alarming but might not cause piggy any trouble. It is always correct to get the vet to check it though. Unfortunately I don't know any more details as nipple lumps are one problem area my pigs have managed to avoid (goodness knows we've had almost everything else!)
Has your boy lost weight? Was it the medicine he choked on or syringe food - this is what caused my initial confusion. Sometimes pigs feel ill and lose some weight and people mush up the pellets (or get some powdered recovery food) and make a slurry to help them with their appetite as it's easier to eat and gain some strength to fight their illness. Sometimes a pig has had surgery or an anaesthetic for another reason and they are slow to recover - again feeding a thin mush of food with a syringe can help. So you can see I got a bit mixed up!
You care so much for this little fellow and he must know how you are concerned for him
He must be feeling a bit poorly and the heat doesn't help. There is a guide on this forum which you may have found about hot weather and guinea pigs - here is a link
Hot Weather Management, Heat Strokes and Fly Strike
Over here in the UK we only have short periods of very hot weather so a popular method for tackling this is to put water in a plastic bottle (only 2/3 full) and freeze it, them wrap in a towel in the piggy cage.
We don't syringe water in hot weather to cool them down as this doesn't really work, however, if we are worried our pigs are ill and might be drinking less than usual we sometimes soak lettuce in water for it to crisp right up before it is eaten. Or if you can get cucumber this is very popular and the soft middle especially will hydrate piggy. But we would advise against syringing water - I have made a mistake like this myself and my piggy inhaled it and would have possibly developed a lung infection except she was already on antibiotics for the thing that had stopped her eating! The medicine was only a small volume - something like 0.2ml antibiotic so this amount is not too much for piggy to handle. But a pig fighting the syringe will struggle with anything over 0.5ml at a time and sometimes even that seems too much for them - large syringes are hard to control I know. Now I make a slurry with the pellets but just make it thinner with more added water if I am worried about a piggy dehydrating...
Keep a close eye on this coughing because if it started when he was fighting against the medicine he might have inhaled some. So he will either get over that or he might risk a lung infection and that could need another vet visit. I'm not an expert but if he were mine I would watch carefully thinking that this could go either way in the next few days. You can perhaps count how many times his sides go in and out to count his breaths. I'm not sure there is an average - my nervous neutered boy breaths
much faster than my confident girl - but if your little fellow's breathing gets faster, or he takes shorter breaths, or he starts to struggle with breathing and you see his sides heave with effort this can point to a developing lung infection and a vet visit with antibiotics are needed.
Good luck, little India piggy x