My dog was bit by some kind of bug?

My chihuahua was rummaging through some bushes and jumped. A few hours later the little guy had a big red spot on his belly. Another hour later a blister about an inch radius but only slightly raised appeared on the skin. Im thinking it was a was sting or something. I gave him a very small amount of benadryll. He was lethargic all day (11am to 7pm). Around 7 he started perking up. This was expected though, as benadryll usually wears off after 8 hours. I rubbed lanacane on the infected area and he seemed to like that. Today though he is still being lethargic, i think it pains him to walk as he tries to itch the area every time he walks. Yesterday he was trembling alot which is unusal for him. Hs heartbeat is a lot slower than normal. He doesnt want to play with toys. He is drinking and eating though. I take him out to pee regularly but he doesnt walk around at all. He usually just stands in the place i put him down and just pees there. He looks alert but the spot hasnt gone down at all. Any help appreciated.