Running and weight gain

I’ve been strength training on and off for about 10 years, started going heavy and focusing on the gym for the last 3.5 years and I’ve built considerable muscle. My week was generally 4-5 strength sessions per week and 1 cardio session (usually bike ride or doing stairs).

I wanted to get into running this year with a group at work and my husband has been training for an ultra. I made it my goal to be able to do 10km. I’ve consistently been running now 2-3 times per week, with one 10km in there, and gym 3-4 days a week.

While my running has improved, the scale has also gone up 3-4lbs and doesn’t seem to come down. I track my calories and weigh my food. I added a 100-150 cals on my longer run days but thats about it. I don’t even think that would be enough to reasonably have truly gained weight over a 2 month period.

Is it all glycogen storage? Water? I’ve been going pretty hard lately so I’m testing out a deload and no hard runs this week.

I’m 5’3, 21% body fat and usually around 123 lbs, for reference.