"I was in the seat next to someone with conjunctivitis does this mean I am going to get it?"
Conjunctivitis is an infection of the tissues which surround and support the eye. 90% of these infections are viral and they do not fly through the air (are not aerosolized). Sitting next to someone will not cause you to develope conjunctivitis. Touching your face after coming in contact with the infection will transmit this infection to your eye.

"Is all conjunctivitis contageous?"
There is bacterial and viral conjunctivitis and these are contageous.
There is also allergic conjunctivitis and this is not contageous.