Why am I so big!?!?

I'm only 10-11 weeks! My belly is already so firm! I hide my bump because I don't want people thinking I'm just faking a belly so soon.. They though I was having twins at first and at a second US they said looks like only one.. I'm starting to question.. I just can't figure out why I'm so bug already :( I'm heftier but have never had a belly out front like this!