Bleeding brown blood okay?

I am 4 weeks pregnant im pretty sure and have been bleeding brownish reddish blood for 3 days-not alot but l have to wear a pad.l also have bad cramping.l went to the er yesterday where they did an ultrasound but they said l wasnt far along enough to see anything and told me its a waiting game.I didnt think its implantation as thats only meant to last a day.Should l worry?