Help Wanted: My hair gets poofy after i wash it.?

i need some help. i was going to wash my hair today but then i remembered after every time i wash it, it'll stay down flat for a while but then when i blow dry it, it gets super poofy! like almost an afro. Then after i would try to straighten it or use my flat iron it doesn't even stay flat all the way. And i cant go to school with my hair more thick looking than it already is. my hair is very thick and almost down to my shoulders. This is really bugging me. Can you help ?