Pretty big level drop - falsely hidden in stats

It's not deceptive. You weren't really level 3.

If you give away a game before its bundle date, you still receive full CV.

Sometimes, it's Steam's API messing up when some items are on sale.

Sometimes, a game's base value drops therefore so does the accordant CV. This affects everyone equally.

It's not worth storing every change in level for every user just to display a graph, so it's extrapolated based on the current contributor value of your giveaways.
For the same reason you will never see a drop in the whitelist and blacklist graphs.

I lost 120+ cv too, not sure from what. Doesn't seem to be the usual suspects.

Total value of all items you've given away = $246
Because they are all bundled games you then multiply that amount by 15% = $36 - which is your real CV
Sometimes freshly bundled games take a little while to reflect the CV drop.

No point in arguing any further.

My big issue is the graph in my statistics

Yup, that graph is pretty BS as it doesn't really reflect your level progression, it only reflects your level progression given the current value of games.
As someone else mentioned, it's the same with the WL and BL graphs. So those graphs are basically just here for decoration, not for accurateness.

is there any tool for getting info about your real Level (eg. for OP 2.44) before you will make another giveaway & Level update?
(not the current Level shown on top (eg. for OP 3.74 before giveaway)

As noted above, it's not deceptive to begin with, as it is properly reflective of the values of that period. The only deception was on the user's end (in claiming to have a level higher than they actually had), but that deception was, presumably, entirely uninintentional on the part of most users.

Of course, if you're assuming that bundle listing is retroactive, then I can see why you may have come to a different consideration.
However, do note that you won't ever get your giveaways dropped down to bundle or free value, if they reach that status after you've given them away; In that circumstance, they would retain full value. The only time value drops by list mechanics is if the giveaways you made were for games that hadn't properly been updated on the site yet (as bundle and free lists have to be updated manually, which can understandably lead to delays).

Well, the one exception to all of this is when base retail value drops, but that's something to take up with the publisher. There's no feasible way for the site to be able to keep track of those kind of changes, so there's no grounds for attributing any kind of intent to that element.