purposly skipped period using BC. cud have been preg from before. now spotting; preg or normal?

i have been taking birth control for about 6 months now & havn't had any problems with it. i am very careful not to take the pills late or to miss any. about 3-4 weeks ago, i was taking the sugar (reminder) pills during the week that i was suposed to get my period. i got it that week but it finished a few days before i was supose to start my new pack. within those 2-3 days, my boyfriend and i had unprotected sex (i hadn't started my new pack yet AND we didn't use a condom). he pulled out, but we were a little worried that something could have happened. now, about 3-4 weeks later, i was expecting to get my period but ended up having to skip it by starting a new pack instead of the sugar pills because i was going on vacation. so now, instead of seeing whether my period would skip or not, i ended up having to skip it on purpose. everything was going fine, until about 5 days into the week of using the new pack when i started getting light spotting. i didn't think much of it. now, however, my spotting is getting heavier and heavier. it's not the same color that my normal period is (it's also much runnier). i have started having really bad pain & cramps, very similar to period cramps but lower down (more in the pelvic area). i also find my lower back hurting really really badly whenever i lie down. my breats feel very tender and sensitive, i have a loss of appetite, i get really bad sudden headaches, and am starting to feel sick at random times. i took a pregnancy test today but it turned out negative.
am i experiencing all these things because of trying to skip my period and my body is adjusting? or was i pregnant and is this the start of a miscarriage? should i change the kind of pills i'm taking?