Hi back. There could be a number of reasons for your back pain. From what you've said yourself, the most obvious things would be connected to your work. In such areas, it's important to learn to bend, lift and protect your back. Instead of bending over, bend your knees and squat but keep your back straight. When picking things up, again, bend your knees and not your back. Takes a little getting used to but will help protect your back from pain and damage. In regards your period. This could also be a causal factor. The only way to be sure is to keep a diary of incidents of back pain and look for any connection. If it is only happening around the time of your period, then a pattern will emerge. For this a warm compress, anti inflammatory meds (ibuprofen) and a warm bath can help. Hope the pain eases soon. P. Either way, it's a good idea to protect your back in your workplace and avoid possible damage.
The only thing I can add to PJBs advice is if you have constipation, as it tried to move down the descending intestines/colon you can feel pain towards the back. I have IBS-C, and years ago that's what my doctor explained was happening. I would actually hurt sometimes across the low back as if I was about to have a period. Warm compresses or heating pads helps only with the pain. If you think it's constipation you can use a stool softener and drink lots of water. If severe try using a glycerin suppository. I have to drink citrucel every day.
If it is related to your job try some over the counter ointment to rub in. You can get ThermaCare pain wraps too. I old the ones with the magnets. There's a new one out there that's like the TENS stimulating pain relief, that enjoys pulled to block nerve pain receptors.