knocking and shuddering

I go faithfully every 3000 miles to get the oil changed, had the serpentine belt replaced about 3yrs ago, and not long after had timing chain replaced. I bought it about 4yrs ago and it was a one owner car with ALLA the paperwork from every repair, tune up and oil change. Now experiencing coolant issues, having to add it on a regular basis; no real problem. I drove it today and while at a traffic light, the check engine light started flashing and it was idling roughly. I put it in neutral and it was fine. The light changed and I put it in drive and it was knocking loudly with no pickup so I pulled into a shopping center lot and parked it. No other lights came on, no smoking, no other noises before this happened.