Experiencing extreme water retention after first hot yoga class

I went to my first Bikram yoga class yesterday. I felt great afterwards and the long-term pain/discomfort in my right knee has noticeably decreased since taking the class, so I am definitely going to continue going regularly.

However, this morning I have noticed that my body is retaining a lot of fluid and I feel quite puffy and bloated all over my body. My weight is (and always has been!) very consistent but when I woke up this morning and felt like I was retaining a lot of water I decided to weigh myself to confirm this, and I am almost 4 pounds heavier than I usually am due to the water retention! I can’t help but be slightly alarmed by this and therefore am wondering whether this is normal for a beginner to experience and whether it will go away as my body gets used to exercising in the heat? I tried to drink a lot of water before and after the class and also had a couple of rehydration sachets to replenish lost salts. Is this water retention normal for people who are new to hot yoga? Will it go away quickly? I aim to practice 3-4 times a week in order to not put too much stress on my injured knee and to give my muscles time to recover in between classes – I haven’t done any exercise (other than walking which I do a lot of) in well over a year due to the problem with my knee so I need to ease myself into it slowly.

I would really appreciate some advice on this if you have a spare few moments.

It may be worth adding that I am naturally very slim (5ft 7in and 110 lbs) and am not starting Bikram to lose weight! But I don’t want to gain weight, either (unless its muscle of course!).

Many thanks.