I wnt flaccid in the middle of having sex. How concerned should I be?

Check with your doc. Erectile function depends on many physical, emotional and lifestyle factors. Losing an erection is something that happens to almost every male at some point, but it can be a sign that you should have the health of your blood vessels checked (through checking bp, sugar, cholesterol & sometimes hormone levels). Regular exercise & avoidance of tobacco and alcohol are essential to sexual health.