I had that when I was about 20 wks. I went to the hospital because my friends mom scared me and said it may be preeclampsia but the doctor just said it was round ligament pain and that I was fine. But mention it to your doctor, you want to be safe than sorry
I had the same thing at 5 weeks. If it continously stays on one side, consult your physician. They will need to see you if they feel it is an ectopic pregnancy. I pray that is not the case. Best wishes