What Causes Nausea, Chills And Dizziness After Taking Plan B?

hi, i started my menstrual cycle yesturday and its heavy for the first time since ending birth control 3 months ago, also i had to take a plan B contreceptive pill 4 days ago. I haven't had much of an appetite today and also forgot to drink water until about 9:15pm today, about 10min after I became severely nauseous and felt as tho i was filling up with air and my stomach started to cramp up. My symptoms have improved with gradual water intake and a hot bath, but they are still come and go. as well as being a little shaky and light headed.