Ikarus, not so much advise, just some academic background info.
Depression is categorised as a mood disorder by the DSM (Diagnostic and Statistical Manual for Mental Disorders) Depression is categorised into 2 sections: 1) major depressive disorder-which can last for several weeks or months, and dysthymic disorder-depressive symptoms that are not so severe. The DSM list the following symptoms for a major depressive disorder.
1)loss of interest or pleasure
2)eating and weight disturbances
3)sleep disturbances. This is a good indication, either waking up 2 to 3 hours before usually, difficulty falling asleep/insomnia
4)psychomotor changes-extreme restlesness and agitation, but behaviour and reactions may also be slowed down
5)loss of energy
6)feelings of worthlessness or guilt
7)difficulties with concentration or decesion-making
8)recurrent thoughts of death or suicide
Not all of the above have to be present to be diagnosed with depression.
Depression can also be triggered, and is linked to chemical imbalances in the human body related to things called neurotransmitters. Two neurotransmitters that are important in studying depression and medicating it are: serotonin and noradrenaline. The first plays an important role in various forms of behaviour-deficiencies in serotonin have been associated with depression, insomnia and aggression (as a side note: LSD blocks the release and functioning of serotonin, leading scientists to hypothesise that a lack of produces hallucinations-psychotic patients such as schizophrenics have low serotonin levels-which could explain their hallucinations)
Noradrenaline is involved in learning, memory, eating behaviour and mood-which could lead to depression. Anti-depressants boost the neurotransmitter levels, making treatment easier by way of counselling.
All three flatmates becoming depressed at once? Possible, but sounds unlikely, I'm sure it has to do with the gas, as other people here have mentioned, maybe you can go to your GP, have him/her take some blood and have it tested.
If it really is depression due to emotional factors-PLEASE deal with it ASAP. Deciding to see a clinical psychologist is not easy, and some stigma is attached to it. Many people will advise you against it-making remarks such as "they're all quaks." Remember, those people saying such things probably do so because they themselves have problems, but are rationalising their choice not to deal with it.
Professional advise and therapy will provide you with the tools to deal with whatever is troubling you (remember: psychologists don't make your problems go away-they help and guide you through the process of helping yourself-this is something many people fail to realise, thus blaming the shrink for the failed therapy)
DON'T grab the bottle-it NEVER helps.
Either way, I hope you are feeling much better, hope all the aboce is not too much academic dribble? My training is still only theory based, but call me again in a few years!!!