I'm a male, 19, 135 pounds. Ive recently have been working with dumbells around 30-40 pounds each at 10 reps. When I do the first 10 reps it's the normal difficulty. However after a 5 min break I came back and did the same thing however this time it was a lot easier where 50 pounds each felt like the same difficulty as the 40s from before. It happens to me often but I'm not sure why. Is it because I'm warmed up or some kind of trigger in my body that makes me suddenly stronger?

Its because you have warmed up your muscles. You have created more blood flow, giving them a boost. That's when you know they are at full capacity.

Tick

Exactly, you got it its because that first set was essentially a warm up set and with the increase bloodflow to the musclefibers you are lifting with more energy :)

Thanks. What id the best way to warm up?

Any type of dynamic movement will help warm you up as well as a little cardio before hand (im a fan of the rower), but if it is heavy lifting you are going for, most bodybuilder and other heavy lifters recommend a warm up set of weights. Lift heavier on your second set.

Why are you having 5 min rests between sets? By the look at your rep range you are training for muscle hypertrophy, if that is the case a 5 min rest between sets is to long. If you are training more for strength the long rests between sets are OK but lower your rep range to 4-6 reps at about 80-90% of your one rep max and stick to compound exercises if training for strength.