Frequency of Braxton Hicks contractions?

I'm currently 28w6d pregnant with twins. I've been experiencing sporadic BH contractions for the last 3-4 weeks but the past few days they are associated with some breathlessness (not due to pain, just tightening). They aren't consistent but probably occur randomly 4-8 times per day? How often have you felt this and starting at how many weeks pregnant? My doctor says they are okay as long as not patterned or 4+ times per hour, but they still worry me...