Inability to concentrate and memorise.

I am a 24 year old student pursuing competitive entrance exams for further studies. I hav always been and intelligent and top ranker student with a strong memory. But offlate my academic performance is wavering to the point that i myself cannot imagine that its me,who is soo lost. As much as i want to study well memorise stuff and work hard to achieve my goals,i just cannot. Apart from thr academic part, my memory aspect is normal though concentration issue is present. This is becoming extremely distressing for me as i near my exams. Also i am not feeling as social as i used tp be wand mostly prefer to keep myself limited to only my family and home. And something actually does not feel right to me about myself. Please help!! As i near exams, i am becoming more anxious. Sleep appetite and blood investigations are normal.