Brown discharge/blood day after sex

Had sex last night. Woke up this morning and went for a wee and when I wiped there was brown blood/discharge. Wore a pad all day and nothing on it but each time I go the toilet and wipe the brown stuff is there on the toilet paper. Had mild belly pains all day. Anyone been like this or know what it is? I finished my period on the 2nd of this month and had sex on the 3rd and 7th and no brown stuff but sex last night on the 13th and now brown stuff today on the 14th. Not due back on till the 25th of this month.