If I went to ER for severe abdominal pain twice but was told that I had constipation and was sent home both times but a few days later my intestines exploded and now I have a colostomy bag for life, do I have a case?

Medical malpractice is negligence.  Negligence is the failure to exercise due care that degree of care that a reasonable medical practitioner in the community would have exercised under the same or similar circumstances to prevent foreseeable harm.
If the diagnosis of constipation without further tests is what a reasonable medical practitioner in the community would have done under the same or similar circumstances, then you wouldn't have a case for malpractice.
If a reasonable medical practitioner in the community would have done additional tests on your intestines, then you would have a case for malpractice.  If the medical records/report from the surgeon who installed the colostomy bag attribute the cause of your intestines exploding to the misdiagnosis at the ER, then you have a case for medical malpractice.
If based on the above you have a medical malpractice case, notify the ER hospital's insurance carrier in writing that you will be filing a medical malpractice claim.
Obtain your medical bills, medical reports and documentation of wage loss.  Your medical malpractice claim filed with the hospital's insurance carrier should include those items.
Compensation for the medical bills is straight reimbursement.  The medical reports will document tha nature and extent of your medical condition and will be used to determine compensation for pain and suffering, which is an amount in addition to the medical bills.  Compensation for wage loss is straight reimbursement.
If the case is settled with the hospital's insurance carrier, NO lawsuit is filed.
If you are dissatisfied with settlement offers from the hospital's insurance carrier, reject the settlement offers and file a lawsuit for negligence against the hospital.
If the case is NOT settled with the hospital's insurance carrier, your lawsuit for negligence against the hospital must be filed prior to the expiration of the applicable statute of limitations or you will lose your rights forever in the matter.