I agree with beerab, ask your doctor what they recommend. There's certainly no reason for you to start eating to gain weight until you know for sure you're pregnant. Even then, pregnancy only requires an extra 200-300 calories per day, but no extra at all in the first trimester. I'm sure you're taking your vitamins which is what's really important in the first tri.
Definitely talk to your doctor, they know all of the specifics of your situation and whether exercise would have an impact on your ability to conceive via IVF.
All that said, I'll be thinking about you and praying for you today! I hope to hear good news soon!
"It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly..."