I could feel "tightening" from about 15 weeks, around the same time i first felt movement. THink it was because i sat at a desk all day so the waistband of my trousers was always over the "bump", making me notice more.

Having said that, as early as 12 weeks i would wake up and my uterus was a hard lump above my pubic bone, which would soften once i'd peed...

Hana