The original claim that "Period cramps can hurt as much as a heart attack." can be attributed to a statement made by Dr. John Guillebaud, a licensed doctor and professor of reproductive health. He originally spoke to Quartz, a business oriented news organization, about period pain and other aspects concerning reproductive knowledge.
Firstly, the claim is somewhat wrong as Guillebaud originally said that, "...patients have described the cramping pain as "almost as bad as having a heart attack.".
Guillebaud did not say that the pain was AS BAD as a heart attack, but close to it.
The actual claim about the level of pain experienced from period cramps being comparable to a heart attack is true, but there is not a whole lot of evidence other than personal feeling to support this.
To begin, the author speaks from her personal experience with period pain, saying that, "Every month I spent hours lying on the floor, unable to move, and literally crying out in agony. My hip and back muscles went into spasm, so that my body was twisted in an S-shape contortion whenever I stood—a condition that didn’t disappear when my bleeding ceased, but had to be treated with visits to a physiotherapist every four weeks." She also describes how friends and family had similar, yet diverse experiences with period pain of different levels.
Based on the fact that the author and many friends have felt such immense pain, there is some truth to the claim, but there is a problem in proving it.
As discussed within the article, there has been little research done on the causes of such immense pain and there are still a lot of questions surrounding the issue.
While doctors currently know that "There are two main causes of period pain: Primary dysmenorrhea and endometriosis.", they do not know a lot more than that. There are a lot of statements within the article discussing the unknowns of period pain. It's stated that "...the medical reasons for primary dysmenorrhea are largely unknown." and that "The specifics of why some people suffer more than others is not well understood."
In conclusion, it is true that the pain of period cramps can reach unbelievable levels reaching up to the pain of a heart attack due to the fact that there are many personal accounts of such pain, but the scientific explanation is lacking due to a lack of research into the subject.
Article: https://qz.com/611774/period-pain-can-be-as-bad-as-a-heart-attack-so-why-arent-we-researching-how-to-treat-it