One of my girlfriends did her IVF cycle at the same time I did my FET. She had an hcg result of 18 at 9 days post a 5day transfer. She had her first BT on the Friday - and then had another BT on the following Monday - so three days later and her hcg was 104. She is currently 21 weeks pregnant. I've heard a few stories of low hcg coming good.

It is all about how it doubles so while 33 sounds low - it may be fine for you and your pregnancy based on your next result! Hang in there and FX you get good news at your next test.