What Causes Menstruation Twice Within A Month After Sexual Intercourse?

I ve come on my period twice in one month only after having sex. This has happened twice...once in jan and this month, too. In jan I passed a large white tissue substance...this has never happened. Then this month, just like in January, I came on just10 days after my monthly...I had cramping for a few days and then a pinkish discharge followed by bleeding. Two days later the bleeding stopped then started again the very next day...my usual periods are 7-9 days and this bleeding lasted 6 days and it wasn t very heavy...what could be the cause? Could it be miscarriage? In jan the pregnancy test came up negative and I haven t taken one this month yet bc technically I wasn t due for my period until the 29th...and this go round the discharge smelled like metal of some sort...any idea?