Hello Gulrina, welcome to UE!
"I am swimming" is the usual form of the present progressive tense:
1. I am swimming across the English Channel today.
"I am doing swimming" is correct if "swimming" is a school lesson or scheduled activity:
2. I am doing swimming, French, and Maths on Monday morning.
In #1, "swimming" is part of the verb. In #2, "swimming" is a noun.
Student or Learner