What Causes Hardness Of Abdomen And Movements In Pelvic Area?

I have implanon and it will be a year expired this month, i had unprotected sex dec 20, now i have pregnancy sympthoms. Im startin to gain only in belly, belly is starting to get hard, and now im feeling movement right above my pelvic area. I took a pregnancy test and came out negative, can i still be pregnant? This would be my second pregnancy and i remember feeling my first chikd move at around 15 weeks. But i was also 75 lbs heavier too.