21 weeks pregnant and braxton hicks contractions after very little physical activity?

I am 21 weeks pregnant as of yesterday and for the past few weeks I have noticed that the slightest physical activity makes my entire stomach tighten up. I am assuming that these are braxton hicks contractions. This is my second pregnancy and I have a 2 1/2 year old son. These occurrences usually happen in the evening when I am playing outside with my son. I usually just walk along side of him when he rides on his toy tractor. No pushing, pulling, anything strenuous. Is this normal? The hardness and tightness usually subside when I sit down for a minute or two but pick right back up as soon as I begin walking again.