Thank you for the query.
Red bump after some time of shaving is caused by hair follicle inflammation
due to its obstruction. Pus starts to accumulate what gives pain and redness
. Hair can get outside the skin and starts to grow inside.
Some people are just prone to such bumps. Hormonal
changes can also cause such bumps.
I suggest you to wash this area before shaving and also do it with sterile shaver. Try to keep it clean and dry after shaving as well. If you will notice that a hair starts growing under the skin, try to puncture it with needle.
Hope this will help.