If you moved it outside during the hot sun, it very likely will get sun burned, as the leaves are not adjusted to the intense light of the sun. This has to be done gradually. The tropical ( house) hibiscus actually does much better outside than inside. They LOVE the sun, but have to be acclimatized gradually.

Your plant must be in a large pot in order to take 2 litres of water a week. Hibiscus like to be watered, but they do not like to have wet feet. They must have good drainage. I have several of mine in large pots, and water every 3 or 4 days, allowing them to get dry between watering. They do well with this treatment.

Your best bet, is to let it dry out, and then get back onto the watering routine that it is used to. Be sure to spray for bugs when you bring it back indoors. Spider mites and aphids love hibiscus, and your plant can become contaminated quickly.