What Causes Abdomen Cramps And Brown Discharge After Taking Plan B?

I started to have unprotected sex with my boyfriend, but he pulled out before he went in even an inch. I took the Plan B pill only a few hours later because I know you can get pregnant from precum and whatnot. My period is irregular, and I usually don't get it every month. However, I started getting cramps, brown discharge, and spotting today, less than a week after I took the pill. Is this a side affect, or is it a sign that I'm pregnant? My last period was from March 24th-March30th.