Thanks for the query to H.C.M. Forum.
May be following causes .
1 Pelvic inflammatory disease
, diagnosis can be confirmed by vaginal swab examination .As there is profuse discharge so we can't rule out any inflammation.
2 Cervical erosion with uterus fibroid
may be the next reason ., diagnosis can be confirmed by PAP smear
3 S T D as you stated in your query that one month back had sex and these symptoms started 3 weeks ago , so after 7 days of sex i.e. may be S T D . Diagnosis can be confirmed by VDRL
In my opinion consult a gynecologist and get his opinion.
Good luck. Dr. HET