Flea/tick/heartworm meds?

My pup is on Comfortis,once a month pill and a heartworm pill once a month under $24.00.He had a reaction to the liquid you stream down the back.Vomiting, diarrhea,lethargy would not wake up when shook or called his name. He was 8 weeks at the time.He is 3 months now.BTW: Checked my records was First Shield Trio he had adverse reaction to.