Although it is known that bikram yoga can cause harm during pregnancy, it is unlikely to have an effect in your case as you only attended a class.
The problem with bikram yoga is not because of the yoga per se, but because of the heated environment in which it is done. The increased temperatures causes decreased blood supply to the fetus and harm it. But it usually takes more than one class for this effect.
I would still recommend your concerns with your gynecologist during your next ultra-sonogram visits are the best modality to check for fetal well-being.
I would also recommend the following-
1) Continue at least 30 min of aerobic exercise or yoga, as it has shown that it improves the maternal and fetal outcomes.
2) Make sure you do not do any form of exercise or yoga lying on your back as it can put pressure on the uterus and reduce the blood supply.
3) Get all the prenatal tests done. Also take Tab. Ferrous sulphate and Tab. Folic acid.
4) Maintain a healthy diet with lots of fiber and water.