Many people feel bloated in the days before and at the start of their period because of rising and falling hormones, which cause fluid retention and can slow digestion. It often comes with mild cramps and a feeling of heaviness.
Helpful steps include cutting back on salty and processed foods, drinking plenty of water (which actually reduces water retention), regular gentle exercise, eating fibre-rich foods to ease sluggish digestion, and limiting caffeine and alcohol. Over-the-counter pain relief can help cramps; ask a pharmacist what is suitable for you.
Period bloating usually eases within a few days. See a doctor if bloating is severe, lasts well beyond your period, or comes with very heavy bleeding, intense pain, or a noticeable persistent change in your abdomen, so other causes can be ruled out.