User blog comment:Ixarising/Speculations, theories and predictions/@comment-26821288-20160511202425/@comment-24.113.252.213-20160512011204

Since Issei was constantly surrounded by women who loved him and tried to gain his attention and affection, Rias was worried about becoming just another woman to Issei and "becoming an afterthought."

Basically she was insecure about what she meant to Issei. Same as in the Light Novel, just in a different context.