Why is my recently neutered male cat growling? Is he in pain?

My fiance & I have a female cat about 1 year old named Izzy. We don't have children as we both have demanding careers so she is like our "daughter". We moved about 3 months ago & our previous roommates had a spayed female cat & they would play & snuggle all the time. I'm not to sure of her age, probably around 2-3. We recently agreed to adopt a co-workers male cat, who is a little over 1. Before we introduced the two, we got the new male cat, named Guy to be neutered. Well I picked him up from the vet last night at 5:30 pm when I got off work & he was of course still really groggy, so we as the vet recommended & put Guy in the guest room by himself with a blanket, his own litter box, food & water. We checked on him every hour until we went to sleep & he seemed to be just fine, just a little sleepy. This morning when I woke up I let him out & he was fully awake. Guy & Izzy didn't get along right away, but Guy showed no signs on aggression at all. No hissing, no swatting, his ears weren't pinned back. He would just follow her around the apartment & try to sniff her, he even tried to rub his face against her a few times, when she wasn't allowing him to get near her he would lay down on his belly & watch Izzy. Izzy was the one who seemed the most unsure. She wouldn't hiss unless he was right up in her face but she did try & swat him a few times. Guy just kind of ignored her moodiness. When I was getting ready to leave for work Guy was licking his & he was growling at the same time. Why is he doing this? Is he growling because he's in pain? If so, what can we do for him to make him more comfortable? While my fiance & I are at work he's in the guest room by himself. I don't know if the kitties will act differently while no one is home to supervise. Is he showing signs of aggression? I hope not I really want to keep Guy I don't want him to have to go to a shelter, but if he's showing signs of aggression I don't know if I'll have a choice. I've tried calling his vet but I've only been able to leave a message. Please help! Thank you!