Thank you so much for your replies. It is exciting to hear from you all.
I have just bought myself an annual card at the Bikram studio where I practice! So I am sticking with it for at least another year. 😉
Electrolyte pills could probably help, I will look into it.
For now, I will pick the coolest spot to practice and accept that I do get weak and do as good as I can manage. I think the key to Bikram yoga is to have a regular practice.