Have you had your bloodwork done recently? It may be a good idea to have thyroid panel/T3/T4/TSH and other thyroid related tests done.
When Lithium level bloodwork is done (and it should be) your pdoc should include Thyroid function workup too.
Your symptoms may be hypothyroidism starting, which while on Lithium is not unusual, especially if you are female.
Some symptoms of hypothyroidism is:
fatigue
weight gain
brain fog
hair loss
joint pain
slow heart rate
dry skin and brittle nails
depression
menstrual irregularities
constipation
feeling cold or chill easily

Just to name a few. After experiencing many of these myself, and later having thyroid ultrasounds, it was discovered and confirmed I have Hashimoto's, once proper dose of thryoid meds were established many symptoms went away or just became more tolerable. It also helped a little with depression.
I think it is important you contact your pdoc and discuss how you are feeling and what you are experiencing, a.s.a.p. it's really not good to play around with med dosages.
It is important your pdoc "listens" to you and works with you on this, if he or she isn't it may be time to seek a new pdoc.
Lithium, like many mood stabilizers/anti-convulsants, can be an excellent med when the doc and patient work together on type of treatment plan, it's also important the patient keeps the prescribing doc informed and up to date with any changes/possible side effects, this way he or she can adjust your meds or even change to an alternative.
Good communications mean so very much in obtaining good outcome.
I wish you lots of luck and patience with this.
Please take care,
DE