Hi and welcome to the forum. It is possible that the active ingredient within your Asacol (Mesalamine/Mesalazine) is causing a rare side effect, but I am surprised nothing was found within the tests that were run.
First possibilityMesalamine-induced cardiac (heart) hypersensitivity reactions myocarditis and pericarditis do occur in 1% of patients taking it and it leads to discontinuation. I've read about
it from time-to-time over the years. You're fairly new to Asacol and that's usually when it occurs, within the first months of use. It's medicine induced, only occurs while taking the medication, and ceases when discontinuing said medication. I'd just bring those up to your doctor and ask if they have indeed been ruled out. I'm no doctor, but what you describe sounds kinda like it. They're rare side effects, and your doctor may not have had this occur before, within his/her practice. So, it's good just to remind them, and point it out on the rare side effects list of the medication you are taking.
Pericarditis is a swelling and irritation of the thin saclike membrane surrounding the heart (pericardium). The most common symptom is sharp, stabbing chest pain that may travel to the left shoulder and neck. Pericarditis usually begins suddenly but doesn't last long.
Myocarditis is inflammation of the middle layer of the heart wall. Symptoms include chest pain, abnormal heartbeat, and shortness of breath.
Here's a couple articles on it:
/www.hindawi.com/journals/cric/2015/728310//www.ncbi.nlm.nih.gov/pmc/articles/PMC2943131/Second possibilityHave them run some bloodwork on you, if they haven't already. Mesalamine can cause wacky blood results and impact the liver and kidneys in some also rare incidences.