Its really normal for symptoms to come and go. And lucky you if you only wind up having them the first 7 weeks! If it eases your mind, give your dr. a call. They can always test your beta/hcg's which, as far as I know, is the biggest indicator at this stage that all is well. Good luck to you, and congrats on your pregnancy.
Same thing happened to me at 9 weeks and I freaked out! I'm 18weeks 2 days now and so far so good! So hang in there and try not to stress out.( I know easier said then done.)....Oh, and the breast soreness came back, the other stuff stayed gone.
I think everyone is different when it comes to symtoms and some will be worse than others. This is my third pregnancy and my symtoms with nausea left me fast I still have the sore breast from time to time , but I believe that is normal. My first and second pregnancy I was sick forever with nausea and I don't think I had sore breast for long ...but like i say pregnancies seem to vary. I think your one of the very fortunate ones to have symtoms leave fast I would not worry much at all. I too first saw doctor @ around 8 weeks and then started testing from then on. Congrats on your pregnancy and enjoy yourself as much a possible hun!