Michael Lohan is experiencing mounting health issues … ’cause TMZ has learned he had a major scare after undergoing a heart surgery.

Lindsay‘s dad tells TMZ … he had just had a stent placed in his heart after he had been feeling extremely lethargic and was experiencing shortness of breath for weeks. Just as he thought he was in the clear, he immediately had another medical scare.

Michael tells us this entire ordeal started a couple of weeks ago when he was folding clothes and passed out in his house. After a series of tests, doctors discovered a blockage in his heart which required a stent.

Michael tells us he thanks his supporters but he’s grateful he made it through.