I just had a failed IVF as well. My RE said I could start right away but my DH and I decided to take a break for a little while. I think if you request an u/s that would be a good idea to determine if your ovaries look good (no cysts). It depends on the outcome of an U/S to see if you can go right into another cycle. Some woman can and others have to wait. Good luck.
My first IVF failed in May and my doctor wanted me to have one natural cycle and then start bcp. So I was scheduled to stim meds two months after the BFN. But I had a cyst and I waited another months so I am halfway through stims right now. I think most docs want you to have a natural month to get all the hormones out of your system and let your ovaries settle back down.
I just had a chemical pregnancy 8/17 and my RE put me back on the calendar for Oct. So as soon as my hcg goes down and I have an af I will start bcp. I think the reason we are waiting until Oct. is because nobody can predict when your hcg levels go back to "0"...She said it was totally up to how I felt...But different clinics have different protocols. Good luck and hope to see a BFP from you soon.