Everyone has good points here. Any one of them could be hitting the nail on the head or it could be a combination of things.
BCC are not as forgiving as
BCI with husbandry.
Have you ever seen or dealt with snake
mites? look closely for them and if you find them refer to the caresheets section for any number of ways to get rid of them.
If as hisba suggested, there are internal parasites, then you will likely notice other symptoms...weight loss, dark runny sometimes with blood usually very smelly stool. But do not assume that if there is normal stool that everything is fine. Take a stool sample to your vet. If there are untreated parasites and the snake continues to regurge he/she (I noticed your snake had a sex change half way through your original post lol) WILL DIE. Make sure the snake is well dydrated before any worming though. Puking dehydrates and dehydration is a death sentence to any young boa. put a 50/50 mix of water and pedialyte in the water bowl even if you dont see symptoms of dehydration. If you do see skin flaps forming along the tail then you need to tube or get a vet to tube some full strength pedialyte a couple of times a day. Do not attempt feeding a snake that just puked for 2 weeks. The acid generated to puke kills gut flora and they just can't digest anything for awhile and will likely just puke it up again making things much worse.
90 is a bit too warm and overheated snakes will soak to cool off but so will parasite ridden snakes.
Mites irritate the skin and internal parasites make their butts itch I suppose.
One prey item for a young
BCC is the only way to go.
BCC are much more delicate as youngsters than any of the
BCI ssps. Feeding multiple prey or prey that is too large for them will sometimes cause regurge.
Now the big problem... Many times when a boa regurges a couple of times, even after whatever caused it to regurge has been corrected, they will for no apparent reason continue to puke and it doesnt take but 3-4 pukes in a row to kill your snake. While a snake can survive for several months without food. They cannot survive even a few weeks of puking every week. They lose too much fluid and enzyme and every puke is like a whack with a shovel.
My last question: How are you thawing the f/t rat pinks?
Do not microwave!
Do not leave them out to thaw on their own
Do not feed f/t prey to a snake that is not fully thawed and warmed slightly (gently squeeze the rats head and feel for coolness. If the head feels cool then the brain is likely still frozen)
The best way I have found to thaw them is to place them in a zip lock bag and run warm water over them...but don't do this too long or too hot or you will cook them.
Some people use a light bulb to warm them..This dehydrates and starts smaller prey to putrify but using a bulb to warm a thawed larger rat seems to work ok if done correctly.
Recap:
Common causes of regurge for
BCC
Parasites
Temps too warm
Temps too cool
inadequate humidity
Prey too large or multiple prey items
Feeding too frequent
Prey not properly thawed or perhaps freezer burned
Stress...Make sure there are hides on both the cool and hot side
Handling after meal (same as stress really)
Here is a tidbit about parasites such as roundworms, pinworms, hookworms etc... Q- Neither parent has worms and the cages are sterile...so how did the babies end up with worms? A- while a well kept, healthy snake may test negative for worms and never have them bother them, most snakes if not all still have them. You can eradicate worms from the gut but you can't kill the larvae that have burrowed through the skin and ended up in muscle. These parastites are stuck there and cannot for some reason make their way back to the digestive system but can penetrate into the bith canal and infect unborn offspring.
[addsig]
272-1330 
