my iphone4s no sign of charging

Hi, need some help.I tried to charge or plug my 4s in my computer i do usually get the 'trust or not' pop up, but i havent now its as if the computer has not even recognized that a device is plugged in. quite strange though I've been checking my phone and the charge is going up by a 1% every 30 minutes or so. but no charge symbol on my phone or notification to say that there has been a device plugged in to my computer- strange.help pls