Hi speedball,
Thanks so much for taking the time to reply. Before I noticed that you had responded, I worked on my shower some this morning. Good news: I fixed it!
I got curious & took the whole works apart. When I disassembled the single handle control, I noticed that it had a plastic piece which controlled the temperature. It was set dead in the middle, hence the lukewarm water. I set it to the hottest setting available, reassembled, and the hot water temperature is now perfect.

I think the problem seemed to be getting worse because it's getting colder outside and that cool air was lowering the temperature a little in the pipes before it reached the shower. I could be wrong but that's my guess. I'm just thankful it wasn't something a lot more serious.
Thanks again for replying. I would have tried your advice next.
