Our 4 Week 0ld Rottweiler Puppy Seems To Have A Pink Blister On Her Anus. What Could This Possibly Be?

It is very, very uncommon for a puppy of this age to need anal glands emptied.  Also food allergies are very unlikely and should not cause blisters on the anus. 
Have your puppy examined by a veterinarian to determine the cause of this blister.  A rectal prolapse is a possibility--and can look very pink in color like a blister.  In a young puppy some kind of congenital abnormality is possible. 
Trauma or injury can cause skin lesions in any dog.
Make sure she is defecating appropriately. 

My puppy is 4 weeks old and since the past week she is running a high temperature ....just a few days ago I noticed a reddish blister under her leg which was the cause of her lack of movement .....that blister burs ted and created a huge whole which is being treated .....her fever reduced too but just today I found a similar blister more on the pinkish color appear around the same area ...am really stressed out since I have no clue how is she getting this ...I have got a blood test done which showed some infection but its cause is unknown ....my doctor said 1st its an incest bite then changed to may be the mother bit her but this seems serious and its spreading quickly plz do help me as am loosing all hope .