I had a similar cramping in my first trimester, My doc said it was due to bubs growing and ligaments stretching. Around 28 weeks I had similar cramps, just a few every hour or so, over the next 2 days the cramps were more persistant. Not worse but just seemed so last longer and gradually increased in how many I was getting per hour.

I wasn't too concerned as there was no discharge, bleeding, back pain, etc but called my midwife and went to hospital to get checked out. At the time I was still working 40hours a week on my feet for 90% of my shift, the midwife explained I was doing too much and this was just the bub's way of telling me to relax and take things easy. I sort of knew I'd been trying to hard to keep up at work and this was the perfect reality check to s-l-o-w down.

I'd recommend talking to your care provider if you have any concerns, if only to put your mind at rest.