I think feelings of depression in general are possible at any time, and having such a young toddler can be very frustrating when your partner is out at work all day.
Infact I remember feeling exactly like this when Zane was that age. I was working 4-5 days a week, and you would think that I would look forward to seeing him on my days off... I really DID look forward to it, but then when the days came, I always just wanted him to go back to sleep or I felt like asking his SIL to come and get him for a couple days etc.
It is a difficult age, because you can't leave them be to play on their own and unwind yourself, but you can't really take them out all day either.
I know some people don't like LO's watching much TV, but I got through it by starting my day with a half hour of cuddles and cartoons on the sofa. Followed by breakfast (which would take him the better part of an hour if feeding himself lol) and then maybe some jigsaw puzzles, reading or scribbling or if he wasn't in the mood for activities we would just watch a little more mickey mouse and I would point out all the colours and shapes. Then he would have an hours nap (he always had an early nap, just maybe 2-3 hours after he woke up to begin with) then by that time I'd had some time to myself and was ready to get him ready and go out just for a nice walk into town - maybe down to the park, but normally just the walk in general was really good bonding time as we didn't use reins so I would constantly talk to him to keep his attention and teach him to stay by my side.
Anyway I digress... my point is, you don't always have to do major activities. Just spend a little time unwinding and have some calm time with your toddler, do things on a slower pace just go for a wander around the block and let him look at flowers etc. If mummy is feeling tired and down, bubbah will feel down and probably wind himself up more.
If you feel like none of this will help, you really should tell your partner how you're feeling... letting go of the secret will probably help more than you realize, and I'm sure he will support you - after all he has chosen to have a family and spend his life with you, he must be willing to take the good and bad x