When a cardiologist inserts a stent, he has to decide on the pressure to inflate the stent to sufficiently push the stent against the artery lining, making a tight fit. If the pressure is too high, the stent puts a strain on the artery and it can become weak. Also, if there is a piece of particularly hard plaque, this when squeezed again the artery, can cause damage. I assume the 'bulge' is scarred tissue because it is on the inside of the artery, and they wanted to balloon it. This makes me assume also that maybe the stent is the bare metal type which doesn't prohibit scarred tissue growth. However, what concerns me more than anything, is the fact that these stents have been in situ for 5.5 years. Surely any strengthening would have occurred by now. If they haven't resolved by now, what makes them think they will?
If it was my heart, I'm not sure I would believe what they say about low pressure and healing etc. I think I would go to my GP and say I want a second opinion and ask to see a cardiologist with a lot of experience. If your vessel started to tear when he inflated the balloon, I'm led to believe the stent is perhaps too expanded and straining the vessel too much. I know if it was me, I would seek a second opinion to make sure that the vessel is not under too much strain.