Sounds like sciatica.
Though I am not a trainer, I do practice yoga, and I suffer of lower back pain, too.
It is often one sided.
I do Virabhadrasana, Ado Mukha Svanasana, and Baddha Konasana, to relax the hip joint. Most of the time, the real cause of low back pain is the shortness of the hip flexor muscles (iliopsoas)
Of course, your description is breef, so it might be something different. If it’s sciatica, you should rest until the pain lessens, than start easy exercises.
Stretching the front thigh muscles, strenghtening the abdominal belt also helps a lot. Do not sit more than 30-40 minutes in one session, stand up, walk with an erect back, try to walk for at least 15-20 minutes daily.
Do not force forward bends.
So if your problem is what I think it is, all the above will help you. But you will know it better … just pay attention to the signals of your body. Do not force anything. Maintain the postures only for 20 seconds, than rest, relax the muscles, massage the streched parts, than try it again. If you feel acute pain, stop at once.