Gaining weight since I started the gym

Contradicting right! I started the gym about a month ago and I was under my bmi weight (not in an unhealthy way ((I'm 16))) and since I started going I've noticed a tiny bit of tone up and some stamina improvements. However I had to weigh myself today and I noticed that I had put on a stone since last month! I understand that I was (slightly) underweight last month and muscle weighs more than fat but it's a bit disheartening noticing that I've put on weight. I was wondering if this is normal and I shouldn't be worried as I'm new to the whole gym thing. Thanks!