Fertility decreases with age, especially after 35. You may gain up to 5 pounds of when first going on certain brands of pills. This is from water retention caused by the progesterone in the pill. This does go away after 2-3 months. The pill has no calories and calories make you gain weight.
Some women experience an increase in appetite when on the Pill.
An increase in food consumption is the real reason for weight gain on the Pill. Very, very rarely, the Pill can affect metabolism. This is easily corrected by changing the pill formulation. The pill is not effective unless used as directed. Taking the pill after intercourse will not keep you from getting pregnant except for use as an Emergency Contraception.