Is this reason to go to an emergency care center?

I am 18 weeks pregnant and I have a doctors appointment soon but for days upon days I have had excruciating pain in my lower back on the left side around my waste and I had trouble sleeping last night because I had a throbbing pain in my pelvis. My lower abdominal has hurt just not at any comparison to the other pains except it's wierd because it comes in sharp jolts. Should I wait until my doctors appointment later this week or go ahead and go to an emergency care center? I figure going to on of those is a little better than the emergency room...