If you have not been on any birth control for the month and have been having unprotected sex multiple times in the week , then this light brown discharge with mild cramps could might as well be an implantation bleed as the chances of conception are high. Though this is a retrospective diagnosis after confirmation of pregnancy, it is advised that you may perform a blood serum beta hcg test on day 22-26 of your menstrual cycle to look for raised levels of hcg , which if present would confirm pregnancy.
An implantation bleed can last for 2-3 days but remains from spotting to light bleed and not like menstrual bleed.
I hope i have answered your query in detail,
wishing you good health,
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