Hello Jlee_lily, and a warm welcome to the forums.
What you have said to us, is absolutely, there are so many different reasons why all of a sudden we begin to feel this way, whether it's slowly been building up and initially, we don't realise it's happening and believing we're only having an 'off day' and be better tomorrow, but tomorrow may not improve, so deeper and deeper we become more upset or perhaps depressed, but this could happen in many different forms of mental illness, but we may not actually know why this we feel this way.
This can cause us to cry over the smallest outcome or even an ad on TV, maybe even a cartoon and can't explain to anybody the reason why we feel this way, we just can't provide any reason and actually, we don't even care at all.
This vicious cycle produces negative thoughts that once before we had never thought of and may only keep multiplying as this illness takes over you, that's why we need help to try and understand why and how this could ever begin, so please seek medical help as well as staying on the forums.
Hope to hear back from you.