On any open sores use hydro cortisone cream to stop itching. If any should look yellow and infected then to the vets you must go. Here One other skin problem that I have recently come across is that of hormonal problems, the symptoms are similar, but always in a heavily pregnant sow, in which case just the cream is often needed. Only boars get this problem and it is when the muscle walls behind the anus stretch and dropping gather in the void. It is going to be a lifelong problem and the lump must be expelled by the owner by gentle squeezing.
Some people find affected boars improve when they are running with sows or other young boars.
If there is no success then go to your vet immediately. Constipation This is unusual in guinea pigs but does happen, droppings are very dry and often joined together, or just non existent. Colds When you suspect a cold, hold your cavy up with its sides pressed to your ear, if it sound wheezy the a vet is needed for the reliable anti biotic Baytril. If however you piggy just has a snuffle, then any decongestant rub, such as vicks, on their chest often does the trick. One note of caution, if at any time your cavy seems to be 'gasping' for air,or breathing very fast, immediately go to your vet as it may be pleurisy, unfortunately I personally have never known a cavy to recover from this.