muscle vs. fat

I got pretty frustrated last week that I wasn’t losing hardly at all, and my roommate and her fiancé (fitness nuts) say that I have probably gained muscle & that’s why my weight loss hasn’t been as steady.  I does make sense a bit, since I have been working out nearly every day. And when I say working out, I don’t mean taking a walk around the block. I ellipticise with my heart rate around 140-150 for 30-40 minutes ever day, followed by 20-25 minutes of lifting weights or doing calisthenics like sit-ups. I’ve had a couple of coworkers say I’ve lost weight, and I think I look slightly leaner; at the same time, I don’t trust my own opinion.