Hi Purplemumma,
I just read on another website a post just like yours and she was worried about a molar pregnancy, a lot of girls came on and reassured her if it was a molar it would be more like 180,000hcg and your nothing like that. This girl went on to have a scan and there was only one bub. Hcg is individual to each woman, some get a really high one and it is only one bub. You would be suprised how many came and posted they also had a very high hcg and there was only one bub. Dont stress, the scan will tell you soon if one or two but it is a possibility like you said that you already have been expecting twins before so perhaps you are again. But just letting you know it can also be just one bub.