Thank you for your query at DoctorSpring.com
I understand your concern.
Yes Flomax can cause lower back pain. I will list out all the side effects of flomax. In case you have a combination of these factors you need to discuss about flomax with your urologist :
Mild dizziness, drowsiness, headache, nausea, diarrhea, back pain, insomnia, reduced sex drive, difficulty in ejaculation, e.t.c
If the symptoms persist you need to discuss about using this medication with your urologist.
Hope this was helpful,
Feel free to discuss further.