Since the pregnancy test is negative it is less likely that you are pregnant and specially with copper IUD in situ there are
The breast symptoms you experience may be due to hormonal changes,frequent urination
might be due to urinary tract infection
Regarding light periods it might be due to hormonal changes or stress related,I would advice you to go for few blood tests like CBC,ESR,Thyroid function tests
,urine routine and micro test and a sonography
of the pelvis.
Meet your gynecologist with the reports.
Don't worry, Take care.