Hair loss on the base of the tail, legs and rear

Hello, I have a cat and she is 3yo and every time she gets heated her fur falls completely, and she would not use the litter she would do it every where. The area that is effected most are legs, rear and the base of her tail, we also tried every type of meds for fleas and they will not go away. I took her to the vet they didn’t do anything they don’t even have the option to spay her and we are also having financial difficulties . So please Help:( what do I do 

Hello and thank you for this excellent question.  It is not unusual for cats to lose hair in that area from scratching due to fleas.  However in this case which looks severe, I would strongly expect there to be both visible fleas and scratching, or at least related trauma (scratches etc on the surface).  Usually, hair shafts are broken half-way down in these cases (it happens secondarily to scratching), rather than the hair simply not growing at all.  Another cause of hair-loss is a hair-follicle mite known as demodex.  These do not usually cause scratching (although their secondary complications might).  However, I have rarely seen this sort of clearly demarcated symmetrical pattern resulting from Demodex in cats.  Usually, when it occurs, Demodex is secondary to some underlying illness.  Ringworm may be a possibility but tends to look more 'mothy' than this, where as in this case the baldness is clearly separated out and your vet may feel that this is less likely.     Differentials (other possibilities) include  paraneoplastic hair loss (this is serious - it's hair loss resulting from cancer) or cutaneous (skin) lymphomas (a type of cancer).  Cats can also lose hair as a result of hormonal changes, which can also be connected to womb cancer, but commonly to womb changes such as ovarian cysts.  In this case, you report that the changes follow your cats' female cycles, which makes me want to take these womb-related possibilities seriously.   I cannot accurately diagnose your cat from here and I cannot sex them from these slightly blurry side-on pictures, so it's worth mentioning, that in the rare case of intersex cats (those with both or mixed male and female characteristics) this is also worth bearing in mind.  I'm afraid that flea treatment will only treat fleas from that list, and would urge you to ask your vet about payment plans or charitable funding in case there is some other way in which you can access further investigation, spay or treatment for your pet.  Because they have the full history and context, they will have some idea of what is best to try first.