Hello and welcome to HCM,
Lifting of heavy weights can put put a pressure of recently operated site.
However, lifting of weights is not likely to cause brownish yellow discharge.
A brownish discharge can be due to passage of altered blood vaginal.
sexual intercourse could have disturbed the surgical site and thus caused bleeding.
However, yellow colored discharge is more in favor of infection.
Thus, I suggest you to get the discharge examined micro-biologically for the presence of infection.
The discharge can thereafter be subject to culture and drug sensitivity to select the most appropriate antibiotic effective against the infectious bacteria.

Thanks and take care
Dr Shailja Puri