Pregnant or nah? Or am I far enough past my fertile days?

Courtney • Love the Lord & all four-legged things. On a mission w/ Mr. Benjamin Robertson since 11/03/12
SO HERE IS MY DELIMA- 
Last night my boyfriend and I of almost 3 1/2 years had unprotected sex. USUALLY use the pull out method. Well, this time, we tried pulling out and my body seemed to just get stuck on him. I know it sounds so stupid, but I don't understand how this happened. 
SO WITH THIS BEING SAID: I HAVE TWO QUESTIONS-
1) Why did that happen? Why did I get "stuck" on him? (It felt like my whole body clenched up after climaxing and I couldn't make it off in time.)
2) With that also being said, he DID ejaculate in me. It happened yesterday, and I was 6 days from my next period. Today I am five days away from my next period. I know there is always a possibility of pregnancy, but can anyone tell me what my chances of being pregnant are? Should I be concerned or am I too late in my cycle?
Any advice helps, so thank you in advance to all the wonderful ladies helping me out. 
If you need any info: I'll be 20 in July, my period is fairly normal, and I am not on birth control as my body can not handle excessive amounts of progesterone without the estrogen to balance it out. 
ALSO: I really don't consider plan B an option.