I believe they auto charge you at the end of the month but I'm not 100% sure.
Yes your credit card is billed for whatever you use at the start of the following month. So if you spend 101$, and had a 100$ credit you will get charged 1$.
wtf - they just terminated my instance for no reason ?? Why would that be the case ?
yes if the price goes above your limit for even 1 second..
They also used to terminate spot instances if you rebooted them, not sure if that is still happening (i never reboot mine). Best to always setup then create an AMI right away once things are working (and setup to automine), then spot launch the AMIs directly.