I take it that youve had your 12 week scan and all was well?
The hormones that your body produces in pregnancy cause the ligaments and bones to soften slightly to allow for the sudden growth that your body is going to undergo. Your back will take on more fat as well as the pregnancy progresses to ease it as you put more weight on.
I would say that if you were standing all day and mixing then its entirely possible that this is just a normal side effect from overdoing it.
If in pain, you can take paracetamol unless your midwife has stated otherwise, its usually perfectly safe during pregnancy.