Ectopic experience?

Has anyone here had any experience with an ectopic pregnancy? I'm starting to worry if I should go to the ER or something. So, around 4 weeks, about 1 day after missed period I had brown spotting. This lasted for two days. Then I started to have cramping but only on my left side, very very mild. Barley noticeable, but my OBs office didn't like the sound of that. So at 5 weeks they scheduled me for an ultrasound. They saw nothing. No sack anywhere yet. They told me to come back in a week and they would look again. Well now I am having sharp sporadic pain in my left lower back not unbearable just there, and very noticeable. I do have an 8 month old I tote around on that side often so I was wondering if it was a pulled muscle, but now there is this odd sensation radiating from my cervix down the vagina wall from time to time. Like a pulsing? Does this sound like ectopic systomps? Should I take a trip to my local med express or something? Also my HCG numbers have been doubling just fine.. 😓