Lower back ache is not uncommon in the second trimester and might persist for a few weeks to come. Its due to the change in centre of mass due to the increasing size of the fetus and inadequate posture maintenance.
Its also due to the stretch of ligaments and hormonal changes so mostly its physiological.
Next Steps
Whenever you're sitting, consider a chair with adequate back support or add a cushion in lower back and avoid slouching.
When standing :
Keep these principles of good posture in mind:
Stand up straight and tall.
Hold your chest high.
Keep your shoulders back and relaxed.
Don't lock your knees.
When you stand, use a comfortably wide stance for the best support. If you must stand for long periods of time, rest one foot on a low step stool — and take time for frequent breaks.
If pain persists you can use maternity support belts but they are not advisable since they cause more compression of stomach.
When sleeping you're obviously sleeping on your side and you can use some cushions to support your knees, abdomen and back as well.
Health Tips
While low back ache alone is not worrisome, it shouldn't be accompanied with any vaginal discharge or burning urination. That might point towards a UTI/ PID or other issues.
In any case if you feel the pain is increasing or is not adequately relieved with these measures you should consult your Obstetrician for further advice.
Take care and stay healthy !