YES I had the same thing. The IVF Clinic told me that i was preg but levels were very low and to come back in a few days to have another (They told me not to get my hopes up). Well, it was still heaps low a few days later, but it had doubled. As long as the levels are going up, doubling or trippling, it is a good sign. Try not to worry! Good Luck
BTW preg all went well for us and now have beautiful DS. And I know EXACTLY how you are feeling right now, in fact i have happy tears for you. I was also told at the 6week scan, that if there is a heartbeat, the preg has a 95% chance of going thru as a norm healthy preg. Fingers crossed for you