For more than two years now I've been doing cardio/weights five days a week. I've had a heart rate monitor for over a year, and work out within my range (125-163), though my rate DOES go higher quite often (never above 190 and never for long).

For the first time today, my heart rate took FOREVER to go down when I got done working out. It just felt like something was "off" the whole time..it was like I was working out in sand. My legs felt heavy. I barely got the whole 30 minutes done, and afterward, where my heart rate usually falls to below the minimum for my workout pretty quickly, after more than 5 minutes it was STILL 150+.

Kinda freaked me out. What was this...have any of you had this happen?

From what I understand, and I researched loads when I first got my heart rate monitor, if your resting heart rate is higher than normal one day or doesn't return to normal as quick after your workout then you may be overtired or overtraining. Dehydration can also play a part I understand.
I know some days my heart rate goes slightly higher than normal and I do tend to find my workouts harder even though I'm not actually working any harder.
Maybe think, did you have enough sleep, have you been doing more workouts than normal or working harder than normal recently, have you been drinking enough water, have you been allowing yourself a rest day each week etc?
If none of these are of any help and you don't feel any better in a couple of days then I'd suggest seeing your dr just to check nothing else is wrong.

I drink a TON of water, but I do always feel kinda thirsty these days. I also get enough sleep. I've always worked out Monday through Friday, then take Saturday and Sunday off....but we did do a lot of yard work this past weekend, so maybe all that finally caught up with me. I'll keep an eye on it for the rest of this week, take it easy this weekend and see what happens...

I was also going to say over-training. I have been told that if your resting heart rate is higher than normal, or your recovery rate is significantly slowed then it is an over-training sign. I have noticed it with myself when I'm worn down.