Okay so im freaking help?!!?

okay i was suppoed to get my period on the 13th. i did not get it.but get this i have NOT had since december and i have had regular periods since then and i knew da*n well i wasnt from then i used a condom.. anyways my cervix is low and hard and closed so i know my period should be coming i have all my normal period signs i always get and i took a first responce test just in case because me and my BF did have *dry * i had my underwear on and was on top of him and he had on boxers and bball shorts. when he came some of his sperm came out the top of his shorts and MIGHT have gotten on my underwear near the top towards my stomach but im pretty sure it cant get through my underwear and pregante me but just to be sure i tested and it was negitive. so i dont understand hwy im late help me im goign crazy i cant concentrate in school. Also i THINK my period chages every 3 months could this be what is happening now? i used to get it on the 4th then in JAN is went to the 13th and back in