Why is my cat in heat after 7 weeks of giving birth?

Your cat can come into heat right after birth--and yours is weaning her kitten, so that's what's gonna happen. 

Start doing some research--there are a lot of no-kill and spay/neuter clinics you CAN afford. There are vouchers available for help when you need them--get on the freakin' phone and get BOTH your cats fixed. It's irresponsible not to. Keep them totally separate until you can. And get some enzyme cleaner--because that un-neutered male is going to stink up your house really badly. 

For the time being, keep those two cats separately so your female cat won't get pregnant. And, the idea to neuter the male one is actually a good one.

Duh! Get the male neutered. In the meantime, shut him away somewhere away from her.  Male cats are cheaper to get done than females.

You need to remove the male cat. Another room outside or wherever until you get her spayed.

"Why is my cat in heat after 7 weeks of giving birth"

Because that's normal in terms of cat reproduction.   They are more prolific than rabbits.  They can be nursing a litter AND pregnant with the next one.

"is there any way i could stop it"

After the kittens are weaned, get her spayed.  If you can't afford basic veterinary care like spaying, you need to give the cat to a shelter so it can be spayed and rehomed to someone who is able to care for it.

Cats are very, very, very prone to have many, many, many kittens.  They reproduce like bunnies.

Your cat was giving birth for 7 weeks? That's one hell of a Mama cat.