Labor?

So I'm 39+5 weeks and this afternoon I've been starting to feel cramps and a slight backache. The cramps only come about every 20-30 minutes and aren't too bad and I notice my stomach tightens when they happen. My upper inner thighs also hurt. Do contractions usually go 20-30 minutes? It only lasts about 30 seconds when they happen. I lost my mucus plug Wednesday and At my appointment Thursday I was 3cm dilated.. I have 2 other boys but it's been 3 years since my last one and honestly I don't remember my contractions starting this way.. I plan on keeping an eye on them tonight but honestly I'm hoping this is the beginning of labor because I'm so uncomfortable and want him to be here already.