Sophie May, Toto's not joking. I've had similar problems too, though thankfully never into the heart attack zone! I was nowhere near severely underweight when I began to experience chest pains of a night, but assumed it was from being "so unfit" and "covered in visceral fat" (essentially my EDs way of tricking me into further weight loss, which has now left me with chronic health anxiety). I had to have a cardiologist tell me that I'm in good shape now, but had I carried on I'd end up with a potentially fatal heart complication. Scary but true.
Right now you're probably safe as it's on the right hand side (the heart's on the left, of course) but all the same, I'm not a doctor and I wouldn't take any chances. The good thing is that as long as you're still able to think and breathe, you can do something about it. Tell someone, for your sake and all of ours. x