My little man just isn't feeling very good.

He had a tick under his neck (throat area) Sat. morning. We tried to remove it, but didn't get it all. I had to take him to the vet to have the rest removed. He gave him a shot that was an anti-inflamatory/penicillin and doxcycline (50 mg) for once a day. This was all about 9:30 AM. Well, late that evening (6PM) we notice his skin inside his back legs were red and blotchy. Called the vet back and said to give him benadryl and call him back at 8:30. The benadryl didn't seem to help, so at 8:30 we go back to his office for another shot. I'm not sure what this one was, but he said he either had a reaction to the penicillin or the tick. This morning the redness seems to be fading, but he's just not spunky like normal. Do you think it could be the benadryl (he had 1/2 teaspoon at 9 last night)? I gave him more this morning and hope to be able to run home to check on him about 2 this afternoon. I'm so sad to see him no feeling well. Any comments or advise?? How about the doxycycline, does 50mg sound right?