Girlfriend is pregnant with someone else's child - suggests an anxiety you have about your girlfriend being unfaithful.

No shoes - This may be a hint at something dealing with your foundation, or basic principles.  Also may suggest you feel unprepared for something in your waking life.

Water - represents your subconscious, your emotions, moods, your accumulated life experiences, and the flow of 'feeling' energy.

Stairs - Going up and down the stairs could mean several different things. It could represent changes in consciousness, movement from one inner plane to another, or a change in understanding. In a more material sense, it could represent a rise or fall in economic or social status and the general efforts that are required to accomplish life's small and large goals. Climbing may represent an achievement of your ambitions and a movement in a positive direction. Descending may symbolize your doubts or a period following hard work and achievement of a significant goal. Generally, dreaming of ascending a stairway connotes movement in a positive direction while descending is indicative of a down period or negative flow of ideas or actions.

Lost trying to exit - In a building that you are generally familiar with, it would suggest you're feeling confusion within your waking life.

Students(random people) - Feeling like you are 'just a face in the crowd'.

Condoms - the obvious connection to the preventive contraception and appear in the dreams that deal with the relationship concerns. In real life condoms are symbols of protection and prophylactics, but in dreams they symbolize psychological protection from all sorts of problems in relationships.
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Has your girlfriend given you any reason why you would suspect her of being unfaithful?
Do you have low self-esteem, and worry that you're 'not good enough' for her?
Have you two ever discussed the possibility of having children together?(Maybe she wants a child and you do not....or not just yet...)
Have you ever heard or seen another man 'looking' at your girlfriend, and you felt a little jealous?

Unless she has never given you reason to suspect her of infidelity, I would say that you're just worried about 'losing' her.

Hope this helps a little, good luck!