Pap smear came back slightly abnormal a typical cells and a yeast infection and I'm 21 does it sound vey serious?

May need repeat pap. Sometimes during an infection, a pap smear may show abnormal atypical cell, you may need to repeat the pap smear after infection is over.
Not likely. Atypical cells on a pap at your age are unlikely to represent severe dysplasia (pre-cancer). It should be concerning enough that you don't skip out on your follow up schedule. For "atypical cells of undetermined significance" i would recommend a repeat pap in one year at your age. If the pap favored a high grade lesion you should have follow up sooner. Check with your doc.