Second baby, rh-, brown spotting before AF was due but now I have a BFP

So I got my BFP recently but have had very minimal brown spotting. First thing in the morning only for two days. AF isn’t due till Tuesday so I figured it was just old blood from implantation bleeding. Well I’m RH- and read that if you experience bleeding you need to go in for the shot. Does implantation bleeding count? Should I go in? I’m not even 3 weeks yet? I plan on calling my doctor tomorrow but wanted to know if I should go to the ER tonight. Thanks!