It could be that you were diagnosed just before starting to feel the effects of hypothyroidism, but that in the time since you started on the Synthroid the hypothyroidism progressed. In which case Barb is right, you are still hypo and undermedicated.
Or, it could be that your body is reacting to something in the Synthroid and causing side effects, in which case you can look into trying a different kind of thyroid med like Tirosint, which is a gelcap and free from many of the fillers that people react to in pills like lactose, dyes, etc.
In either case, please post your test results with ranges so we can get a better idea of what's going on.
It's not a good idea to just stop taking the synthroid. Typically, side effects of synthroid are those of, either under or over medication.
Your symptoms appear to be from under medication, indicating that you are still hypo.
While many people gain weight when hypo, hypo isn't necessarily associated with being overweight. Anyone, any size can become hypo, not just those who are overweight. Just prior to becoming hypo, I weighed less than 110, at 5' tall. Not everyone gains weight when they are hypo. It's not unusual for symptoms to worsen or for new ones to appear, when first starting medication. It can take your body several months to adjust to the medication.
What are your current thyroid hormone levels? You should be getting tested regularly for TSH, Free T3 and Free T4. Please post your latest results, so we can better comment on your situation. Be sure to include reference ranges, which vary lab to lab and have to come from your own report.
Also, what dose synthroid are you on, and was that your starting dose?
Have you been diagnosed with Hashimoto's, which is the number one cause of hypo in the developed world?