I don't think you should be going to the ER for this, your regular doc should see you.
Keep taking your medicine and check at the end of the dosage. If it hasn’t cleared or you’re unsure head back to the doctors and explain. You might need more :)
It has been my experience that clindamycin works super well but sometimes needs a second prescription (i.e., I need to take it twice as long) to get rid of the thing it's trying to get rid of. Let your doctor be your guide, don't try to self-diagnose, but if the white stuff is still there after you finish 6 more pills, call in and tell the doc what is happening.