Hai Rosy,

PGDM expands to Post Graduate Diploma in Management, the confusion starts when you see the term Diploma. Many students are of the view that it is a Diploma course and not a Degree; you are right to an extent that it is a Diploma. The main reason why postgraduate courses are named as PGDM is because when an Institute is an autonomous body (meaning it is not affiliated to any University) and conducts management courses then such Institutes cannot offer MBA degree. Even IIMs, XLRI's doesn't offer MBA degrees they only give PGP Diploma or PGDM since they are autonomous and independent bodies.
When an Institute secures autonomy they design their own curriculum to be taught instead of following outdated University syllabus. The reason why they chose to do is because the curriculum can be updated regularly depending on the need in Industry. Hence one can say that PGDM courses are more Industry relevant as compared to MBA courses.
Colleges which offer AICTE approved courses will also offer PGDM not MBA, for example ABC College is affiliated to Bangalore University they offer MBA course affiliated to that University, the same college is offering AICTE approved course it means they are offering PGDM which is not linked to Bangalore University.
Typically if any PGDM is to be equivalent to MBA then the course duration should be 2 years, a 1 year PGDM will not be equal to MBA course.
Yes, it makes sense to go for PGDM after BMS