charyuop

Hee hee we are the exact copy one of the other. Even my lower back kills me and unfortunately it has very little to do with Ukemi.
A compression of the lower back is normal for everyone and it is much more increased in people overweight like us.
What gets worse is that the compression will get nerves involved and in the long run it will start effecting your legs (trust me at times it gets really painful, worst than cramps). A chiropractor can help fixing the problem (temporary, go to a doctor for better dyagnosis), but if there is a strong inflamation sometimes shots in the back are needed.

Now said that, you can think about your Ukemi and how they afftect your back too (I am talking always comparing to what happened to me). My shoulders where hurting coz my front roll more than an actual roll was a downward drop on the shoulders. So I tried to compensate to save the shoulders and instead of landing on them I started landing on my lower back. Can't even describe how this + my chronical condition made me feel, not only in the morning, but all day long. It was enough to sit for 5 minutes and when I had to get up I would walk bent in 2 coz I couldn't streighten up my back.

First things first. Go to a doctor and have your lower back checked (if it already got into the legs probably an MRI will be done to check the situation). Start working on losing some weight. I use a diet called Provida (there is a site online) which is very similar to a diet I got in a gym in the past. You get all kind of nutriments, you don't lose weight by eliminating nutrients factors that your body needs.
Then I suggest restart with Ukemi from scratch, as before me suggested from kneeling down position. This time the pain already existing will let you know if you are doing it right or not. Absolutely avoid to create a loud noise sounding like a BAM on the mat. Good Ukemi are basically noiselss.

Just 2 cents from a person who has been there before and still is there...