you shouldnt be ashamed its normal. but you can put something really warm on your stomach not just a heating pad can work i mean i use a water bottle filled with warm water also, make sure to stay away from junk food since it will make the cramps worse.
If it is bad take a panadol or two you can also use a heat pack on your stomachs and say you are cold? If you are swimming definitely use a tampon and don't be ashamed every girl gets their period, how old are you?