I've been experiencing tightness in my lower abdomen the last few weeks with mild cramping. i am 21 weeks pregnant, is this a contraction?

Few things: Need to r/o urinary tract infection, constipation as the cause of cramps.As the baby grows and the uterus enlarges under the stimulation of female hormones they may be normal uterine contractions. After all these things, then a possibility of uterine irritability leading to uterine contractions need to be r/o. Improve hydration, avoid constipation with appropriate food , walking , seeing /talking -dr.