The infection could be what is making you extremely tired, but it could also be the thyroid issue.
Goiters can be caused by an over- or under-active thyroid and depending on the person, both can make you extremely tired, either because you've got too much thyroid hormone so your body is on over-drive and you become exhausted (and often can't sleep) or you've got too little thyroid hormone and so you lack energy because of that (and can sleep too much, but still feel tired). I would also say that if the tiredness has been recent, like since you've been sick, it's more likely due to that. But if it has been coming on for a while and you have other symptoms, like aches and pains, dry skin, hair loss, weight gain or loss, things like that, then this may have been a longer-term problem that you just never thought much of until now. Unfortunately, thyroid symptoms can be vague and can come on very gradually, and often people dismiss them as "normal aging" or don't really notice them until they really get bad.
The ultrasound should be able to tell if this is a goiter that is part of the thyroid, a growth on the thyroid, or if it's just a swollen lymph node from your infection. If the lymph node is still swollen after the ultrasound, they may want to do a biopsy, just to make sure.
If you have more questions later, feel free to re-post.