I accidentally took 2 macrobids of 100mg; should I be worried of overdose?

Not at all... The usual dose of this med is 100mg 2x/day thus 200mg per day. I assume you are using it for a urinary tract infection. You u are you and healthy, which i assume you are based on the medication given, you should be fine. You can skip a later dose today, or even taking your later usual dose without trouble. So, rest assured...But just be carefull next time around...Good luck.