hi,
as
spermatozoa can be alive for 5 to 7 days after intercourse in case of ideal circumstances, so the intercourse before fertile period means on 28th may cause
pregnancy,
then after 9 days what you saw could be
implantation bleeding,
or all these may be just inter menstrual bleeds due to hormonal imbalance,
so you once go for
blood test for pregnancy which will possibly rule out the condition.
take care.