always remember that whenever you or anyone who planned to start with exercise, start with minimum than gradually increase it day by day, for example you should start with 15 sits-up then adding 10 sits-up everyday like in your case, never go for sudden heavy exersion its a simple advise.
well with your sick feeling, and light headedness pale lips numbness
might be due to just simple ANEMIA
Hb level in blood needed by the body for proper oxygenation) so you are advised to go for some clinical assesment like PALLORness of your conjuctivae r hand and also go for Hb level for diagnosing underlying anemia if present any.
you may also need some evaluation for cardiac assesment and require enquiry regarding any positive heart disease
family history if present any, to take you on a safe side otherwise there is nothing to worry if all labs and clinical assesment appear to be in normal limit
otherwise keep on a good healthy diet that include fresh fruits vegetables and a first class meat product that boost up your energy level and immunityand maintain your Hb on a good normal range as you are v young adult need good balanced nutritious diet.
hope that helps you