Pressure On Bottom Of The Hip Bone, Nausea After Lifting Heavy Object. Cause Of Concern?

Hi, I m 20 years old and I lifted and was carrying around my 4 year old nephew at the mall. After putting him down Im having pressure on the left bottom on top of my hip bone on my stomach and was feeling a little nausea also.. This never happen before to me and I just want to know should I be concern?? Also I should add in is I was 4 days late on my period and my period finally came and I m on my 4the day on it which it usually last 5 days. But I been having un protective sex, so I was wondering is it a chance that I could be pregnant? That I think it s my period but really it s implantation?? Please help!!!