Actually, with all due respect, synesthesia is not something that only presents itself at certain times; it is a constant condition and does not only manifest itself during a specific timeframe (like during the menstrual period, as the OP describes.)

Also...synesthesia, as you mentioned, is a "cross-wiring" of senses. The five senses are sight, smell, taste, sight, and hearing, so nausea really wouldn't fall under any of those senses. Therefore, what the OP is experiencing is simply sensitivity to a certain smell, at a certain time.

Now, if she was associating the sight of mushrooms with the colour red, or if she pictured the word "mushroom" as being red, then she would be classified as being synesthetic. For example, when I see the word "mushroom", I see each letter in a different colour, because I am a diagnosed grapheme-colour synthete.