worriedbabe answered your question beautifully.
I dont know how much birth control pills cost w/out insurance but you could also use a spermicide in addition to condoms. Condoms do have a failure rate of up to about 5%.
it could be the birth control. if i were in your position though, because of the sharp pains - i would see a dr. periods can be delayed/ missed altogether for many, many reasons. i don't think you're pregnant. however, bear in mind...the most careful of us can still get pregnant, i've been caught out on contraception 3 times. it's a possibility you should always keep in the back of your mind when you're sexually active.