Can constipation lead to bad cramps and nausea ? also can it be a sign i'm getting my period soon ? i usually get it around the end of month

Yes: Yes to both! Constipation and menses can both cause cramping and nausea. Usually there is a difference between in the way cramping feels between the two. Some women get bowel changes associated with their period due to the hormonal changes. I would take fiber and drink lots of water 1 week before your period every month. Use miralax (polyethylene glycol) if needed for the constipation.