Allergies and PMS: weight gain or water retention?

What lietchi said ^^^ endorsed.

Even the allergies themselves can increase water retention (inflammation, plus if it's respiratory as it sounds like, the gunk in your sinuses is largely water, even if it's kinda thick and icky).

The scale isn't making accurate estimates of fat/water/muscle. Even if it's off by only a percent or so, that's multiple pounds. At most, those scales might be good for visualizing a long term trend (many weeks to months), and even then you'd want to ignore weird individual readings that are out of the general pattern). They are absolutely not good for assessing day to day (or few day) changes.

I'd even broaden out the implications of your monthly cycle: Different women have different patterns of hormone-related water weight changes across a cycle, and some don't even have one consistent pattern. When you've been at this for multiple months, you may begin to see what your personal pattern is. Even though it's not the most common pattern, a few women here have said they only see a new low weight once a month!

If a cheat meal has more salt or carbs than your usual - even if a perfectly sensible amount of both - that can increase water retention, too, because our bodies need extra water to metabolize those things.

As a general rule of thumb, multi-pound/kg changes over a small number of days - if there's no change in calorie intake/activity big enough to explain them - is going to be water retention changes or digestive waste in transit on its way to the exit. With a consistent routine of activity/calories-eaten, fat loss is a pretty small number of grams per day. (Muscle mass changes are even slower, in a healthy person - they show up over multiple months at minimum, usually.)

That fat loss plays peek-a-boo on the scale with the faster and larger-magnitude water/weight shifts, but show up in the body weight trends over multiple weeks to months.

As a personal example, I'm trying to ultra-slowly creep down in my maintenance weight range right now.

This past 7 days, I've weighed anything from 129 pounds to 133 pounds - not the high weight at the start and the low one at the end, but those are the low/high points over the 7 days, with other days up and down between those. That's roughly a 2kg swing.

A month ago, in the similar 7-day period, I was meandering still, but with a high weight of 134 pounds and a low of 132.2 (again, not the end points of that week, just the extremes during it).

My conclusion is that in the past month, I've probably lost 1-2 pounds. Since I'm aiming for slow loss, that's perfect for me. That's in pounds, but I think you see my point: Loss on the scale isn't down-down-down, it's up and down day to day, trending down over long periods. (My bodyweight translates to around 59kg, for context - so my loss in a month is somewhere between half a kg and a kg, roughly.)

I'm sure you're going for a faster loss rate than I am now, which is fine with more to lose, so you may see quicker changes in your trend. But I'd expect you to still see that kind of up and down day-to-day pattern (even week to week sometimes), with an overall down-trend over longer times. Because your body size currently is larger, you may see larger magnitude fluctuations than I do. It does vary by individual, but your 1kg is probably a very small percentage of your current bodyweight, right?

Please try not to be anxious. What you're seeing is perfectly normal. If you do the right things with eating and activity, you'll see the good results in trends over a period of weeks.

I say weigh yourself and gain some information about how your body works, if you can do it without feeling too anxious. If you can figure out when your body has natural fluctuations, it can be annoying but comforting. In the future if you know it's not something you did to cause it, that can keep you from becoming frustrated and giving up.