As per your statement, You are obese and suffering irregular period for a several yrs. Recently you have lost some weight. Now you are c/o post coital bleeding, nausea and spotting. All tests done fir pregnancy
Point No. 1 - Regarding irregular periods
and obesity Test your Thyroid
profile and treatment if there is a problem.
Point no.2 - Regarding post coital bleeding - various causes may there - some which can't be taken mildly.
Few causes are:
Vaginal yeast infection:
most of the above mentioned are infectious.
but a proper investigations like USG, Endometrial biopsy
and vaginal smear study may be needed
nausea and loss of weight may be due to the underlying cause.
treatment according to investigation results after consulting a gynecologist