Office politics effecting my work and personal life - how to handle

Wishing all an Advance Happy Independence Day wishes from my end.
I joined an IT company in Chennai as their HR Officer on 1st August 2017. Two more girls joined as developers on the same day. While I was very friendly with these girls, irrespective of the difference in our designations, soon I realised that their attitude was quite different from mine. I found that one of the girls took as many / long breaks as a person on a Managerial level would. She takes long tea breaks of upto 30 minutes, most of which time she spends on her mobile phone. I don't want to be in that group and so slowly I have avoided joining them in lunch timings and continued to have my lunch in my cabin.
I have informed to HR Head about it and he has given me the task of warning that girl regarding the usage of cell phone during working hours. Now the situation in my office is that both the girls treat me as their enemy and have even bad mouthed me to the HR head who seems to believe the girls instead of me. While sending mail no salutation or no thanks message.
Kindly guide me how to change this situation.

"Irrespective of designation"
Please understand HR is not a disciplinary agency or superior to others.
Complaints about late coming, wastage of time is initially with HOD and not directly HR.
Office politics is at play here and mid of your HR boss is biased against you to some extent.
You being too friendly initially and now being distant has caused bad blood.
How is it that the HOD of the employees is not being objected by their HOD as he is primarily responsible for their output.
Ignore the attitude of these employees for the time being.
Sooner or later their time wasting will come to notice of other officials also.
Any action initiated by you in the circumstances will bounce on you as your boss is upset and the employees are not counselled by their own HOD.

For any discipline issue, you should work through HOD and not directly confront the employee as wrongly advised by your HR boss who doesn't seem to know basics.
As an adult , nobody likes to be told or behave even though he is at fault. However unpleasant, it is each HOD's foremost responsibility to enable team discipline and team spirit. Your role as HR is only that of catalyst .
For the time being forget what happened with the truant ladies and your boss and move forward doing your job with clarity and conviction about your role which is well- founded on HR beliefs and principles .
Regards,
Vinayak Nagarkar
HR- Consultant

The genesis of the problem , as I understand , is "I have informed to HR Head about it and he has given me the task of warning that girl regarding the usage of cell phone during working hours."
You seem to have been misguided by your HR head.
It is the problem of wasting time should have been reported to HR head by concerned department,if they could not handle the issue themselves.
For now , suggested course,if to try and forget the issue and lean from this episode

If you had diplomatically handled the situation with the female resources on the spending time on the phone or taking long breaks. You would have not lost your peace of mind, also you would have earned brownie points , informing your Manager on your pro active measure.