Curious to whats going on

Been on a combined pill for about 3 weeks now. Probably had unprotected sex 3 times after the 7 day period of using the pills.... ive been having mild cramping on and off in my lower abdomen and sometimes lower back for the past week I would say and mild headaches more recently. I should be expecting my period Jan 13th. I took a pregnancy test on Dec 31st which came out negative... not sure if my body is preparing for my period, if my body is reacting to the pills now or if im actually pregnant. Help???

(Planning to take another pregnancy test next week)