sudden breastfeeding issue

The past few days my 9 month old has been struggling with nursing. When I say struggling I mean he will latch then unlatch repeatedly all the while becoming angry and frustrated. Eventually he does settle down, but my once 5-10 minute nurser is suddenly nursing for twice as long. Could my supply have dropped and that's why he's acting like this? I did take some allergy medicine could that have caused low supply? Also my husband and I have started trying for number 2, so could that possibly be it?