1980 Other Volkswagen Models fuel burp

After having spent 14 years in storage, I've gotton my Scirocco back on the road. It needed a new fuel tank installed due to excess rust in the original. Our mechanic found and installed an aftermarket OEM replacement that was fashioned after the original (exact duplicate). Everything seems to be fine. Today the fuel gauge had finally gotten into the lower region, so I filled the tank up for the first time today - filled on level ground, pump cut-off automatically with no spillage, nothing extra was put in. Prior to filling up, I had noticed that the car was balking. So after, the fill-up I bought some fuel injector cleaner, but did not put it in right away. I drove home, washed my car, then about 3 hours later went to put in the additive. When I opened the lid (also on level ground), it was under a lot of pressure and blew out about 2-3 oz of liquid fuel. It has never done that before - usually a breif expelling of vapor pressure. What's the problem?