Difficulty Maintaining Erection. Porn?

Hello, I am a 25 year old guy having a fairly okay sex life but I have realized that since the past few days, I am unable to maintain a boner while having sex. I generally am erect and do go on for a long time (40 minutes or above) and that is why this recent stride of erectile dysfunction is making me feel worried. I can't tell what it is but it's not lack of sleep or stress for sure because I'm doing fairly well in those areas too. I'm really perplexed as to what could be causing this and would like to get some help. From some self diagnosis, I wonder if it's the style of masturbation that is translating adversely into my sex life or could it be because of watching too much porn? Would appreciate some help here. Thanks in advance:)