Immunity depends on infection severity. People with mild infection have very less /negative antibody tests.
You will not be immune against covid -re infection as different strains are now found.
You may have some partial immunity against the strain that caused infection this time.
Next Steps
dont hurry back to the gym.
even if you are feeling better, your body is recovering from an Illness.
give it time to recover. start slowly and increase exercise gradually.
post covid myalgia, fatigue is common upto 2 - 3 months hence watch out for that.
Health Tips
improve diet and monitor saturation thrice a day.
hope you recover soon ✌