Women certainly need calcium and vitamin D. Osteoporosis is a major problem in later years, but can be prevented in the teenage years. Taking calcium and vitamin D in your teens (and throughout your life) will protect you from fractures and bone loss in later years.
Don't compromise on the calcium. If it constipates you, take Miralax daily and this should help alleviate your constipation. Miralax is an odorless, tasteless crystal that attracts water to your colon and softens your stool. It is not a stimulant and will not cause you to cramp.