I have the left side of my lower abdomen and tummy slightly bigger than the left side. and i have also had a missed period and bloating and hard stool. and i have cramping in my lower abdomen for about a week now. what could be the problem?

If it's constipation: A young adult with constipation needs to be sure there is no other disease causing it. If constipation is just due to slow movement of food and poop through the gut, then the things to try are vegetables, water, prune juice, prunes, fiber-with-water, polyethylene glycol, Senokot or other senna, milk of magnesia, other laxatives... A doctor can evaluate and maybe prescribe helpful medicines too.