Braxton Hicks at 18 weeks?

For the last week or so I've been getting these tightening like sensations in my upper abdomen, it's like my ab muscles are so tight they might tear! It happens only at night after a long day and only when I stand up...is this a Braxton hicks contraction or just normal stretching due to showing more? Last night they got so intense I couldn't even stand up straight and had to sit immediately after trying to stand. Everything was fine after I sat but my abdomen still felt uncomfortable and tired... I called me OB and they haven't gotten back to me so I was just wondering if anyone else has this.