Thank you for asking the query.
This particular bleeding is nothing to be worried about. Since you just restarted OCP after a break, this could be a form of withdrawal bleeding. The regularly hormones offering the endometrial support might not have reached its target levels, and hence the bleeding.
STIs will not present like this, as you rightly said.
And yes, stress and inadequate nutrition could be a risk factor. Have nutritious diet and you can consider taking Multivitamin and Iron tablets for a couple of months in consult with your Doctor.
You need not do anything actively to stop the bleeding now. It should clear by its own. You need to be concerned only if the bleeding keeps happening for more a couple of times (not including this episode). In that case you may have to visit an OBGYN.
Hope this helps. Please feel free to ask follow-up questions.