Mouse over your influence total (should be in the bottom right) and it will tell you what is costing you influence, and you can react accordingly.
You cannot form an army because while just having an army of yourself costs no influence (you're doing that already), recruiting parties into the army does cost influence.
Yes check your influence income, could be slowly being put into negatives. to get more donating troops or prisoners to a friendly castle nets a pretty high amount.
No, it takes 50 away to propose, and then another 100 because I think it is deducting for if you had voted or something. Doesn't it take influence to vote? I am not saying it should do this, but I think since it automatically does the vote (since it is just you) it takes the influence away. So you lose 150 influence every time you do a policy.
As neppy and BlackRainZ have said, if you have no vassals, policies cost you 150 Influence rather than 50. I assume this is a bug. If you're not in a position to recruit vassals or simply don't wish to, hire one of the mercenary companies when they're available. Even though they can't vote, the vote screen comes up when you enact a policy and the correct amount of Influence is deducted.