I have cramping at 18 weeks pragnant?

I am 18 weeks pregnant. I have been getting period like cramps throughout my entire pregnancy so far, even though the baby's heartbeat and everything else is normal, I am still VERY concerned! It can be extremely painful and very uncomfortable. I actually have to take tylenol or advil for the pain otherwise it will not subside fast enough.(my dr. said that I could take "advil" the 1st & 2nd trimester BUT not to take it in the 3rd trimester). I was also told that this could be caused by not drinking enough water and that the muscles can cramp from this. So, I was simply wondering if anyobe else has suffered through this or has an anwser? Thanx!!!!