Your tachycardia and dizziness could be because of following reasons
1.. You may be having low blood sugar
resulting in hypoglycemic attacks.
2.. You may be having hyperthyroidism
symptoms due to excess of SYNTHROID.
So get your TSH levels checked again to see whether you need revision of SYNTHROID dose.
3...you may be having electrolyte imbalance
due to dehydration caused by taking of SPIRONOLACTONE
4.. Get your BP checked as it could be due to hypotension.
Consult your doctor and get the relevant investigations done and if necessary get your prescription revised.