The gain likely to be mostly water weight from the intensity of your workouts. Our bodies store additional water to repair damage, which is bad and great at the same time.

It's bad because the panicky amongst us race around acting like the sky is falling because we are gaining too fast and rationally we know it's not all muscle and panic it's fat However it's good that your body needs the extra fluids because it's while we are repairing we are growing!! whoop whoop.

I wouldn't worry to much about a sudden gain in the first month, that is normal and expected in the first month, however now your got the 'water' all factored in your inches and weight should stabilise.

Judging how much body fat/muscle you are gaining during a bulk is damn hard without a submergence body fat reading.

All you can do is monitor ... from this point forward you don't want to gain much very fast at all. It's really good going for a none testosterone taking woman to put on an inch of muscle in a year never mind a month. Your measurements/weight should creep up .. slowly slowly to the point where it will drive you crazy because they never seem to budge, same with your weight if you are gaining more than 0.25lb per week then you are likely adding too much fat.

Keep an eye on the mirror you should aim to keep the same leanest in appearance as you grow, you may lose a little definition but if you are getting noticeably squidgy this is a sign you need to dial back the calories because your gaining alot fat.

The best way to judge your muscle gains is with your lifts. If you are consistently lifting bigger and more weight then your growing. Its the only really good way to know..

Personally while doing a bulk I put away the measuring tools and focus on the lifts I go insane otherwise.