spotting reassurance

I just need some reassurance. I went to the washroom today and noticed there was some brown discharge that looked like old blood when cleaning up. I noticed it again when I went to the washroom 4 times later throughout the evening. It hasn't gotten any darker and I just have the usual soreness but no other pain. My doctor said it was nothing and not to worry about it. I had to pee in a cup and then he just came in the room and told me it was implantation bleeding. Should I still worry cause I feel a little sore but nothing i haven't felt before. I am 9 weeks 4 days and I had an ultrasound at 6 weeks and we heard a heartbeat. 

Thank you in advance!