I've been cutting for 3 weeks now and have lost 5 lbs. I don't know if it's a psychological thing though, but i feel like my muscles are flatter, less full and generally I'm looking worse. My strength hasn't decreased, still lifting the same weight/reps as at the height of my bulk, so can I use that a good, reassuring indicator that i'm not losing muscle?
That's usually a pretty decent indicator. More than likely you're just a bit glycogen depleted (the body stores glycogen in your muscles). Besides, it's a bit more difficult than people think to lose an appreciable amount of muscle mass. The people who miss one meal and run around screaming how they've lost size have only shed a bit of water and glycogen. Keep going
if you are losing weight and not losing numbers on your lifts that's a good indicator that you are not losing muscle.
Judging by the fact you have lost "only" 5 pounds in 3 weeks (A great amount for "pure" fat loss, in my opinion) and that all of your strength is still there, I will echo what Stu and Kaiser said.
I will add that If you have been low carbing it for a few weeks, you may want to consider a high calorie/carb day soon to both fill your muscles back up with glycogen, and kick start your metabolism so you can keep on keepin' on with the fat burning.
LOL this is me right now. Tweaked something in my low back and haven't trained in 3 days and I feel like I'm shrinking by the day hahaha