A vet would probably insist in doing a skin scraping, while I never do. I do not have the facilities and as most parasitic problems can be cleared up by using the kind of shampoo used upon children for hair nits, see Skin problems, you can use this and kill two birds with one stone. A good shampooing is always of benefit to the skin, and if it is a parasitic infestation you will have cured the problem.
If the hair does not start growing again, or continues to fall out a couple a weeks after the parasitic treatment then it is very likely to be straight forward alopecia.
The bad news is that I have not discovered a cure for this, but the good news is that in most cases the hair does eventually come back again. It may take anything up to six months or as little as a month. The cause is probably hormonal and there may even be a way of getting it balanced again but I have not even bothered to look into this for this loss of hair has absolutely no detrimental effect on the animal suffering from it.
The reason I have come to this conclusion is that alopecia is very common in pregnant guinea pigs, as it is in their human counterparts, when the lymph system is under stress. The most hair loss is off the rump, though it can be from the whole of the underside of the animal. Usually by the time the sow had weaned her young the hair will be well on the way back. If she has caused lesions by scratching herself I will put a good covering of calendula oil on her to speed the healing of the skin but it, of course, has no effect on the root cause of the problem.