I would not necessarily assume that hiring for a senior position is the same thing as promoting you to such. It depends substantially on your company, of course, but my company hired a senior developer on my team shortly before promoting me to senior developer; here, senior is mostly a way to pay us more money, and doesn't have a substantial change in responsibilities beyond expecting a bit more on the mentorship front. They may simply need another body, and want a more senior body. It's not like you're suddenly going to be a different person after all when you're promoted; the need for someone able to take on more responsibilities than a junior [x] may exist regardless of your promotion.
A consideration though even if this is the case, is how you will fit in with the new person in terms of seniority. Hiring a senior person while you're not a senior, particularly if they then have some degree of responsibility over you, may be uncomfortable, particularly if you indeed do get promoted a few months later; it may be actually the ideal time from your manager's point of view to process the promotion so as to ensure that isn't an issue.
The way to find out, of course, is to talk to your manager.
Hi [name], I know we've been working for the past year to get me to the point that I would be able to be promoted to Senior [x]. I noticed yesterday that we posted for a Senior [x] position. Is this the right time to discuss my promotion as well? If we're bringing a new person onto the team, it would be good to have a solid idea of how we're going to fit together in terms of responsibilities and team structure ahead of time.
That reminds them of the work you've put in to get to a good position for the promotion, without being too whiny. It also gets to the root of the matter: that you want to find out where you are in terms of getting that promotion, and that way if the answer is that you're not ready yet, you can then push for some details as to what you would need to do in order to get there.