Can dehydration and masturbating too much cause my pee to burn?

Yes, but: More likely than ether of those things as the cause is a urinary tract infection and/or a sexually transmitted disease. Drink lots of fluids for a couple of days, stop masturbating just for a day or two. And if symptoms not better or pain increases, fever, abdominal, then it's so easy just to get urine tested to see what the true cause is.