Cold Shoulder by Co-Worker?

My co worker who is also close friend of mine has totally stop talking and ignoring me for past few weeks. We work in a same team and I am at the higher level/position in the team. I manage the operations and will handle more complicated task. However, I am expected to reduce my burden by delegating my task to my next level due to incoming job. I have been behind her back all the time and supporting her to my team lead whenever she unable to cope. I will be assisting her by drafting mail to clients and solving complicated cases. Recently due to incoming new job, I have informed her what to do rather than doing it for her. And I could not spend time monitoring her work anymore. Due to this , she was stressed with deadline and was being monitored closely by my manager. Since then she has stopped communicating to me. All the staff is ok with me except recently another staff who is always with her being sarcastic when communicating with me. I have ignored all this for weeks. But not I feel embarrassed as I know others has noticed this. Being their supervisor it is not healthy to have such behaviour with my co worker. This has actually happened few times but I have confronted her where she back to normal each time but never informed me what went wrong. I do not want to be the person who always letting go and allow this to be a habit. Please help