HI Rosieroo
I started getting braxton hicks around 22 weeks, but they were never painful and there was no pattern to them either.
If you're getting period-like cramps in your back/stomach every 15 minutes then call your midwife (or the weekend number at the hospital) - chances are it's nothing and you can be re-assured, but don't take any chances.
Let us know how you get on
LBxx