I do empathize with your sufferings.
You are having pus discharge which is responsible for your low back ache and discomfort at the private parts. Please get the culture and sensitivity of the pus done. It will identify the causative bacteria and the antibiotics to which they are sensitive. This will enable the Gynaecologist to use appropriate antibiotics and treat your infection. If it is from the Bartholin's gland, you may have to undergo marsupialization surgery.
Once your infection is eradicated, your thorough clinical examination can be carried out to look for any other pathology.
My dear, Dermoid cyst
is not a cancerous cyst
, it is benign type. So you need not be stressed for it.
Now cooperate with your Gynaecologist and get treated completely.
May be your partner also will have to take the treatment. Usually these type of infections are more likely to be Sexually Transmitted Infections.
I hope you got your query well replied to.