Hi, I recently bought a new brand of food for my cat and like always started mixing it in with the remaining old food so he'll be used to it. Just about 3 weeks ago I went out of town for a day and came back to find that he had vomited what looks the semi-digested food. Last night almost 3 weeks later he vomited again, but there was less. Should I be worried about this and if so what should I do about this?

I also had started using the bio spot flea and tick spray, which the application of does coincide with the two incidents

I would think of hairballs first, heart-worms second, allergic reaction third, and lastly what did the stupid cat eat now? I have had all of these apply at one time or another to one of my cats. Because of the time lapse, I would go with hairballs or allergic reaction over the other problems. If you use the bio spray again and the vomiting happens again, stop using the spray. If the vomiting happens and you cannot connect it to anything, treat for hairballs. If neither of these solutions work, it is time to go to the vet.