Gaining muscle just by running

There's no way you've been gaining muscle, you're eating at a deficit, you're doing cardio, and your protein intake is absurdly low. Weight gain/weight loss has nothing to do with what you're eating, and everything to do with how many calories you're eating, and how many calories you're burning.

1) You don't gain any appreciable muscle with cardio.

2) You certainly don't build 2.5 Lbs of muscle in a couple of weeks...if that were possible, every guy on the planet would be yolked. A male with half way decent genetics, eating a surplus of calories with a spot on diet high in protein and working his *kitten* off in the weight room might...maybe...perhaps put on a good 8-10 Lbs of muscle in an entire year.

3) 2.5 Lbs is well within the range of natural body weight fluctuations. Also, if you're new to running it is likely your muscles are retaining water...they do that any time you switch up exercise and/or increase intensity because your muscles need water to repair.

4) It is likely you're also eating around maintenance...as you say, you havent been making the best food choices.

if you just started running, it's probably just your muscles retaining more water to repair themselves

edit: just saw this was already mentioned above

Can you gain muscle from running? No, at least not any noticeable amount.

Can you retain water (and thus "gain" weight) from exercise? Absolutely.
Is this what is likely happening to the OP? Yep.

There's no way you've been gaining muscle, you're eating at a deficit, you're doing cardio, and your protein intake is absurdly low. Weight gain/weight loss has nothing to do with what you're eating, and everything to do with how many calories you're eating, and how many calories you're burning.

That simply isn't true. The types of calories you take in definitely have an effect. It isn't as simple as saying all calories are created equal.

As far as running 20 minutes every other day causing muscle weight gain, I'd have to say there is next to no chance of that. You certainly can change your physique from running and you will look leaner with less body fat, but I don't think you actually gain muscle.

There's no way you've been gaining muscle, you're eating at a deficit, you're doing cardio, and your protein intake is absurdly low. Weight gain/weight loss has nothing to do with what you're eating, and everything to do with how many calories you're eating, and how many calories you're burning.

What he said
Also if you are getting your heart rate up to a certain level, which by running I assume you are, your body starts to act more in a cardio exercise manor (starts breaking down muscle for fuel) then a fat burning manner (exercising at a lower heart rate).

So I know that when exercising, it's possible to gain weight sometimes simply due to muscle gain. I know that happens with weight training, but I'm not sure about cardio.

I've gained about 2.5 pounds these past 2 weeks, and at first I wasn't sure why but I looked back on my food diary and saw I haven't been making the best food choices for the last 2 weeks (it's open if anyone's curious), so that would explain it.

But regardless, I was wondering if it's possibly to gain enough muscle from doing something like running every other day for 20 minutes. I'd like to start weight training soon (I go back to school in 3 weeks and don't see the point in getting a gym membership now when I have a free one at school), so for now the treadmill I have at home and some Zumba is my only form of exercise. Thoughts?

Are you by chance closing in our experiencing your TOM?? A 2.5lb gain in the last week sounds like bloating or possibly carrying some glycogen absorbed by your muscles.