Well there are a number of factors that are possibly causing such chaos.....

Yes it could be your Lamictal tanking. How much are you taking? The half life is 24-43 hours. So if your med sensitive this could be the problem or part of it.

Alzheimer patients often gets whats called "sun downers" its almost like clockwork in the afternoon they get more and more agitated and confused and often mean.

I think Bipolar in many ways is the same..

If your working on the basic lifestyle hours 9-5 so to speak., By late afternoon your just tired fed up annoyed pissed of and generally cranky like a 3 year old that skipped there nap .. Of course that is going to ramp up the negative thoughts and your self worth is going to take a beating, daily .. So your mind gets use to this and you just start a vicious cycle..

How to help matters?

Certainly look at your medications. If your highly sensitive any fluctuation can send you wobbling.

Try to increase any and all coping skills when you first notice your mood starting to turn.. There's typically always a pattern.. watch for clues.. Are you hungry? maybe your blood sugar bottoms out around that time? Have a snack see if that perks you up.. hell bribe yourself with a treat for a few days and see if that helps.

Turn on lights ,open blinds turn on some music, up beat none of that trigger inducing stuff, just don't go there..

Hows your sleep at night? Are you getting enough? Are you getting quality sleep? There is a huge difference between getting 6 great deep restful hours and 12 poor lousy stuff.

Now the last bit of advise and trust me I know this is hard and your going to shake your head, I know I did when my T told me to do this.. Get a jar, pretty or neat or cute , decorate something yourself use a mason jar , whatever... Write down a whole bunch of things you like .. Like .. Summer rain, particular coffee you like , A flower, a book you love , any quotes that you like or are funny, Silly jokes or other nonsense that can be so lame that in its self makes it funny. write down " hop around like a rabbit, Play hopscotch on an imaginary sidewalk , sing kid songs .... Ok, stop shaking your head no just do it ... Pull up shyt on the internet to put on slips of paper, just write out some stuff , fold them up and toss them in the jar.. Pull one out when your hitting that wall, give it a whirl.

And if all else fails and your really losing ground take a PRN,, Better to take them a bit more often than doing a daily crash and burn that is not going to help you at all.. Its much easier to pull yourself outta a gutter than a deep dark well.

Anywho... I hope you find something that will help you fend off a whole bunch of nastiness.. I'm sure I have more lame ways that could help if these don't work out for you.

Just a question... when was the last time you bought yourself flowers? If you had to think when , or have never done so ,,, Do it ! Its a wonderful way to treat yourself well