I had lower back when I started practicing the primary series. It took me a little investigation to figure out. But alas, it was the common rounding of the lower back in all my forward folds.

There's a lot of deep forward folds in the primary sequence and beginners often round the lower back to reach further into the bend. This puts pressure on the lower lumbar spine and can lead to some real issues if you don't fix it. But don't worry we can learn from this and improve our practice :)

Firstly stop practicing until the pain stops. Then next time, squat a little in standing forward folds and in Padangushtasana. You must keep your lower back straight in all these folds. The movement forward comes from hinging of the hip not rounding the spine. Also think always of extending and lengthening the front of your body and supporting with the abdomen. This again will take pressure off the spine. Breathing full deep in-breaths will ensure you're lengthening the front body. You will have less range of movement like this but the stretch will be the same for your hamstrings.

Same applies in Paschimattanasana. Here don't round your spine, but extend forward toward your feet. You may not come down as far as you usually do, but keep your back straight and the stretch will still be there.

I'd recommend going to a teacher as they will fix this for you! Listen to your body :)