Lower back sore, Deadlifts too heavy or body adjusting?

Hi everyone,

So, I was very happy at my 35lb lifting yesterday - but today my lower back is a bit niggly and sore. Is it just soreness from starting a heavier weight and it's normal, or is it a sign I pushed myself too much? I definitely watched my form in a mirror, engaged all the muscles, so it's not form. Any advice/experience offered from when you upped your weights?

Today was supposed to be a recovery day, but had work and couldn't do it - so tomorrow will be recovery session and the following day I'll start my 4th week of alternate Sweat/Strong training.

I should also point out that work was giving an orchestral concert, so perhaps it wasn't much of a day of rest for my body today!

Thanks in advance!