How could mass increase when sulfuric acid is added to calcium carbonate?
There are several contributing factors to this outlier observation. First of all, when scientists did mass based experiments, even hundreds of years ago, they recorded mass up to four decimal places. The mass of CaCO3 should have been recorded to 0.1 mg for example 5.0612 g. When you say that mass increases, we do not know the details of mass change. In short, never record mass with one significant figure.

Secondly, how did you measure volumes of 20 mL acid? If you were using cylinders, they are never accurate. There is significant error in volume transfer. With a relatively viscous solution such as 2M H2SO4, if you transfer 20 mL, the transferred volume via cylinder is usually less than 20 mL.

This all about good laboratory practice. Coming to chemistry: when CaCO3 reacts with H2SO4, if forms an insoluble salt called calcium sulfate (CaSO4). With concentrated 2 M sulfuric acid, once it comes in contact with CaCO3, it may form a coating of CaSO4, which would slow down the rate of reaction. Did you see some unreacted solid mass?

Also conc. sulfuric acid is hygroscopic.