I pulled an all nighter on sunday and have been sleeping more than normal (8 hours) the last couple days but am still exhausted. Mono or REM rebound?

Sleep debt. When we pull all nighters, we build up a sleep debt that can sometimes take weeks to "pay off". Unless you're having fever, major sore throat, and other symptoms of a cold - you probably don't have mono. Keeping getting your 8 hours of sleep, avoid alcohol and get daily aerobic exercise. You'll start feeling better soon!
Sleep debt. Your brain keeps track of your sleep debt and will want to make it up. Many americans have dozens of hours of sleep debt collected which is why when you sleep in on the weekend you may not wake up refreshed - you only payed back a portion of your sleep debt. The average sleep required is 8 hours and 10 minutes. Some need more and some get by with less.
Could be. After any period of sleep deprivation, your body is going to make up the sleep that was lost. It does this both by sleeping more and making up the various stages that were lost. In the beginning, you do get a rebound of rem sleep. Then, you make up deep sleep and other stages. But, you should return to normal after a few days.